Webrender: Implement blur filter

[mrobinson]

This can be observed by running the following wrench test:

root:
  items:
    - type: stacking-context
      bounds: [10, 10, 500, 500]
      filters: blur(5)
      items:
        - type: rect
          bounds: [10, 10, 50, 50]
          color: green

cc @glennw @jrmuizel

[glennw]

It's not implemented yet, that's why :) Most of the work is done in other areas, so it shouldn't be a huge amount of work.

There is a (slow) blur render task and shader that is used for text shadow blurs.

There is also some code to render a stacking context to a target with identity transform.

We basically need to hook up / abstract those bits of code, to get the blur filter working.

We also need to (at some point) write an optimized blur shader that doesn't have awful performance, but that can be done later.

[glennw]

This happened.