Fix text images not fading

While a7b2291 makes text sprites
modulate their RGB values, text images continued using alpha,
despite alpha blending not even being enabled, so the initial
commit didn't work right either.

desktop_version/src/Graphics.cpp

@@ -973,6 +973,7 @@ void Graphics::drawgui(void)
             opacity = tl_lerp;
         }
 
+        const int alpha = opacity * 255;
         if (textboxes[i].image == TEXTIMAGE_LEVELCOMPLETE)
         {
             // Level complete
@@ -996,15 +997,14 @@ void Graphics::drawgui(void)
                     y = 240 - y - 8 * sc;
                 }
                 SDL_Color color = TEXT_COLOUR("cyan");
-                const int alpha = opacity * 255;
                 font::print(
                     (sc == 2 ? PR_2X : PR_1X) | PR_CEN | PR_BRIGHTNESS(alpha),
                     -1, y, translation, color.r, color.g, color.b
                 );
             }
             else
             {
-                const SDL_Color color = {255, 255, 255, (Uint8) (opacity * 255)};
+                const SDL_Color color = {alpha, alpha, alpha};
                 if (flipmode)
                 {
                     drawimagecol(IMAGE_FLIPLEVELCOMPLETE, 0, 180, color, true);