…ical

Opacity is a common effect that is used and the opacit filter path is also used when a stacking
context has an opacity of < 1. The brush_blend shader is slow since it has support for a large
portion of CSS filters; however, opacity is used much more often than the rest of the filters.
This patch adds a simple shader for opacity effects which bypasses the extra overhead in the
brush_blend shader.

Differential Revision: https://phabricator.services.mozilla.com/D59610

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/b0226237c55e4a08f4deb622fc15fa1e85f39afc