There are two fixes here:
 * Image pictures were generating clip masks, but they are never
   actually used. In the future, we *might* want to support this,
   but at the moment it just results in clip masks that are then
   never used.
 * If a primitive is in the root coordinate system, then we know
   that it is axis-aligned, and that there are no coordinate
   system changes in between this primitive and the root. In that
   case, we can infer that the local_clip_rect of the clip scroll
   node will correctly handle the screen space clip rect.

In Gecko with the current WR code, I was seeing a lot of very large
clip masks being generated (4x extra A8 render targets) on HN. The
test case included here is a very reduced test case for these issues.

The test case makes use of the new reftest annotations to assert
that no alpha targets get allocated while rendering the yaml file.