Text LCD loops (both hw and sw) should support rendering into a non-opaque
destination with SrcOver compositing mode.

FX applications often render into non-opaque destinations, and current work
arounds (6749060, 6728834) force grayscale AA in this case. The issue is
compounded on Windows because we switch from the native text rasterizer
to t2k so the glyphs shapes change noticeably.

Ideally we'd teach them to support non-srcover compositing modes (SrcOverEa would
be nice) but that may be harder.