I removed the old opacity filter path in the brush_blend shader and cleaned up the filter mode
constants in the shader so there are less magic numbers. This should help if/when we move more
filters to their own shaders.

Depends on D59610

Differential Revision: https://phabricator.services.mozilla.com/D59611

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/5cd63d35010f9bf9538a37fb330127d459b7ccc0