Partially fix blur filters.

[pcwalton]

• The temporary framebuffer texture is now unused, since we use separate
  render targets instead of readback. This patch removes it.
• We now write the texture sizes into the mask UV coordinates properly.
• We allow composite filters to inflate the dimensions of their render
  targets. This means that blurs can draw outside their rects.

Remaining issues:

• Blur filters need to not read outside of the render target in the
  atlas.
• The vertical blur needs to blur the render target that the horizontal
  blur wrote to rather than the original render target.

[pcwalton]

I'd be interested to know what you think the best solution for the second problem here is. A blur essentially needs to result in the creation of two nested render targets. At first I just tried to special case this in the composite batching code (creating a second render target for a blur) but that seemed inelegant, especially since we'll probably want to be able to batch them (i.e. blur many elements horizontally, then blur many elements vertically). So I figured it'd be better to hook this into the layers/render target infrastructure somehow—essentially to create two nested render targets for each blur. Thoughts on that?

[pcwalton]

@glennw Fixed mix-blend-mode.

[glennw]

Thanks!