Bug 1035611 - Part 1: Updated Matrix4x4::ProjectRectBounds to properly handle infinite values when untransformed rects cross the w=0 plane. r=mattwoodrow

- Added specialized rect clipping functionality to Matrix4x4::ProjectRectBounds
  so we don't have to return infinite values when rects cross the w=0 plane
  in homogenous coordinate space.
- Updated callsites of ProjectRectBounds to pass a clipping rect that is
  appropriate for the units that are returned.

--HG--
extra : rebase_source : 9b1088a77e88259e2c32379ade23fbc2236596fb