* Blur filters do not need a temporary framebuffer. (Nor do any other
  filters.)

* 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.