Premultiplied RGBA support

Some video outputs need premultiplied alpha.

There is a work-around in the X11 RENDER output to multiply RGBA in the X11 server, but that only works with RENDER.

Blocks #12345

added Component::Subtitles Severity::normal Type::feature Version::master git + 1 deleted label

changed milestone to %Features paradize

marked this issue as related to #12345

marked this issue as related to #21020

unassigned @MigrationBot

Any update on this? ASS rendering on android is still broken (and slows down the video by a bunch whenever transparency stuff happens) because of #21020, which depends on this issue.