Refactor texture-related classes

[jfirebaugh]

#5101 introduced RenderTexture and #5190 introduced Texture. There's also #4811, an older take on the same idea by @mourner, and existing code that uses texture-related WebGL calls directly without using either class.

We should refactor and sort this out. Here's my proposed design:

• We need a state-tracking wrapper like gl-native's gl::State<T> and associated value traits.
• However, the way that native does texture binding is not quite right. All that should be publicly exposed are property setters and methods on the Texture class. Internally, a property setter should:
  1. Check the tracked state, and exit if it's a redundant operation.
  2. If not, then check that tracked state of the texture binding for the current texture unit. If changing that is a redundant operation skip it, otherwise bind the texture.
  3. Update the state.
• For texture handling that's not connected to a draw call (e.g. stuff that happens in the upload step), just always use texture unit 0. For texture handling during a draw call, there should be a way of supplying Texture references as the value of uniforms, and having the internals handle assigning unique texture units, binding the textures, and setting the unit identifiers with glUniform*.

[ryanhamley]

Closing as outdated