Tiled gradients with many tiles can render incorrectly

[eqrion]

Discovered from looking at the test case attached to bug 1443912.

The following test case is a red rect covered entirely by a 100x100 tile tiled radial gradient. Each tile of the radial gradient is solid blue, so we should only see a blue rect on the screen.

Instead we see that some limit on tiles is being reached and not all tiles are being drawn.

---
root:
  items:
    - type: rect
      bounds: 0 0 800 800
      color: red

    - type: radial-gradient
      bounds: 0 0 800 800
      center: 4 4
      radius: 4 4
      stops: [0, blue, 1, blue]
      tile-size: 8 8
      tile-spacing: 0 0

cc @jrmuizel

[nical]

This is because of:

webrender/webrender/src/display_list_flattener.rs

Line 2376 in c32585c

max_prims: usize,

where we set a max limit of 64x64 primitives during decomposition.
I'd like to move this logic to the render backend thread and then from there implement out a solution to having many repeated primitives. @glennw suggested prerendering N repetitions in an intermediate target and repeating that M times using a decomposed brush image (but first I need to finish porting the image decomposition to the render backend as well).

[nical]

This has been fixed by #2644 and #2752.