glamor: Properly handle mask formats without alpha.
Even if the pixmap's storage has alpha, it may have been uploaded with garbage in the alpha channel, so we need to force the shader to set alpha to 1. This was broken way back in 355334fc. Fixes rendercheck -t composite -f x8r8g8b8. Signed-off-by: Eric Anholt <email@example.com> Reviewed-by: Michel Dänzer <firstname.lastname@example.org>
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