- fbCombineSaturate was pointed at fbCombineDisjointOver, instead of
fbCombineDisjointOverReverse as it should. Instead, point
fbCombineDisjointOverReverse at fbCombineSaturate (which is likely to
be faster).
- fix previously-unused fbCombineSaturate implementation.
- fbCombineMaskAlphaC was just a copy of fbCombineMaskValueC. Make it do
what it's supposed to (return a cs.alpha).
- fbCombineAtopC didn't invert the source alpha value.
- fix copy'n'paste errors in fbCombine(Dis/Con)jointGeneralC, also source
alpha wasn't treated in a component fashion.
- fbCompositeSrc_8888* didn't handle when the source lacks an alpha
channel. Rather than adding that and possilby slowing down the (normal)
alpha case, don't let x8r8g8b8/x8b8g8r8 Pictures be used in
fbCompositeSrc_8888* because Over with one of these is just Src.
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