Fix static decoding of interlaced or downsampled transparent GIFs.

The GifDecoder was NPEing, forcing Glide to fall back to BitmapFactory,
which can only decode static images.

Fixes #2698

instrumentation/src/androidTest/java/com/bumptech/glide/load/resource/gif/GifDrawableTest.java

@@ -44,16 +44,120 @@ public class GifDrawableTest {
 
   @Before
   public void setUp() {
-    // Required for us to add a View to a Window.
-    assumeTrue(Build.VERSION.SDK_INT < Build.VERSION_CODES.M);
-
     context = getTargetContext();
     mainHandler = new Handler(Looper.getMainLooper());
   }
 
+  @Test
+  public void loadGif_withInterlacedTransparentGif_sizeOriginal_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.interlaced_transparent_gif)
+            .submit()
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
+  @Test
+  public void loadGif_withInterlacedTransparentGif_downsampled_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.interlaced_transparent_gif)
+            .submit(10, 10)
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
+  @Test
+  public void loadGif_withTransparentGif_sizeOriginal_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.transparent_gif)
+            .submit()
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
+  @Test
+  public void loadGif_withTransparentGif_downsampled_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.transparent_gif)
+            .submit(10, 10)
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
+  @Test
+  public void loadGif_withOpaqueGif_sizeOriginal_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.opaque_gif)
+            .submit()
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
+  @Test
+  public void loadGif_withOpaqueGif_downsampled_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.opaque_gif)
+            .submit(10, 10)
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
+  @Test
+  public void loadGif_withOpaqueInterlacedGif_sizeOriginal_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.opaque_interlaced_gif)
+            .submit()
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
+  @Test
+  public void loadGif_withOpaqueInterlacedGif_downsampled_succeeds()
+      throws ExecutionException, InterruptedException {
+    GifDrawable gifDrawable =
+        GlideApp.with(context)
+            .asGif()
+            .load(ResourceIds.raw.opaque_interlaced_gif)
+            .submit(10, 10)
+            .get();
+    assertThat(gifDrawable).isNotNull();
+    gifDrawable.stop();
+  }
+
   @Test
   public void loadGif_intoImageView_afterStop_restartsGif()
       throws ExecutionException, InterruptedException {
+    // Required for us to add a View to a Window.
+    assumeTrue(Build.VERSION.SDK_INT < Build.VERSION_CODES.M);
+
     // Mimic the state the Drawable can get into if it was loaded into a View previously and stopped
     // so that it ended up with a pending frame that finished after the stop call.
     final GifDrawable gifDrawable = GlideApp.with(context)

instrumentation/src/androidTest/java/com/bumptech/glide/test/ResourceIds.java

@@ -16,6 +16,10 @@ private ResourceIds() {
   public interface raw {
     int dl_world_anim = getResourceId("raw", "dl_world_anim");
     int canonical = getResourceId("raw", "canonical");
+    int interlaced_transparent_gif = getResourceId("raw", "interlaced_transparent_gif");
+    int transparent_gif = getResourceId("raw", "transparent_gif");
+    int opaque_gif = getResourceId("raw", "opaque_gif");
+    int opaque_interlaced_gif = getResourceId("raw", "opaque_interlaced_gif");
   }
 
   public interface drawable {

third_party/gif_decoder/src/main/java/com/bumptech/glide/gifdecoder/StandardGifDecoder.java

@@ -602,8 +602,7 @@ private void copyCopyIntoScratchRobust(GifFrame currentFrame) {
             averageColor = act[currentColorIndex];
             if (averageColor != COLOR_TRANSPARENT_BLACK) {
               dest[dx] = averageColor;
-            } else if (
-                isFirstFrameTransparent == null && isFirstFrame && !isFirstFrameTransparent) {
+            } else if (isFirstFrame && isFirstFrameTransparent == null) {
               isFirstFrameTransparent = true;
             }
             sx += sampleSize;
@@ -619,7 +618,7 @@ private void copyCopyIntoScratchRobust(GifFrame currentFrame) {
             averageColor = averageColorsNear(sx, maxPositionInSource, currentFrame.iw);
             if (averageColor != COLOR_TRANSPARENT_BLACK) {
               dest[dx] = averageColor;
-            } else if (isFirstFrame && !isFirstFrameTransparent) {
+            } else if (isFirstFrame && isFirstFrameTransparent == null) {
               isFirstFrameTransparent = true;
             }
             sx += sampleSize;
@@ -635,7 +634,6 @@ private void copyCopyIntoScratchRobust(GifFrame currentFrame) {
     }
   }
 
-
   @ColorInt
   private int averageColorsNear(int positionInMainPixels, int maxPositionInMainPixels,
       int currentFrameIw) {