Factor some parts of the YUV brush shader out into a shared
yuv.glsl shader include.

In future, this shader code will also be referenced by the
composite.glsl shader when using the simple (Draw) compositing
mode, to composite video surfaces directly into the framebuffer
where possible.

Differential Revision: https://phabricator.services.mozilla.com/D63123

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/bde7c898937d877132b5d2aaeae022227c9da7af