…aders with PERSPECTIVE feature. r=jrmuizel

This implements Z plane clipping, gl_FragCoord.zw interpolation, and vertex attribute perspective-correction with support from the glsl-to-cxx compiler by using PERSPECTIVE feature in a WR shader.

Differential Revision: https://phabricator.services.mozilla.com/D69429

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/88a4ebce9de0a0b261747ce7327f26600a551475