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 integrating use of Filter::Mask in Gecko, the implementation of Filter::Mask is bug 1784382.