Firefox uses clipmasks very little (just on tab title text, and rarely on web pages), we should refactor the clipmask functionality into a specific Filter::Mask operation rather than have it woven throughout the WebRender code which is a maintenance burden and a source of significant code duplication.

Additionally, applying clipmasks as a filter in the space of the destination surface will resolve aliasing artifacts in bug 1671784 where a rotated clipmask has undesirable image border issues.

This part of the solution is implementing Filter::Mask, Gecko integration will be addressed in a separate but related bug.

Implementation of the Filter::Mask operation which applies a clip mask
texture when rendering a stacking context into the parent surface.

Added a reftest as a proof of concept as Gecko doesn't use Filter::Mask
yet.

This shouldn't have any impact on existing code paths - integration
with Gecko will be a separate commit.

Status: Nearly looks right with my code changes, but there are polygon gaps between ImageMask tiles at some scroll offsets and zoom levels, so there's something doing some coordinate rounding in the primitive handling that I need to fix.

The following patch is waiting for review from a reviewer who resigned from the review:

:ahale, could you please find another reviewer?

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