Fix problems in render fb implementation found by rendercheck:

- 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.

fb/fbpict.h

@@ -333,15 +333,8 @@ fbCombineDisjointOverC (FbCompositeOperand   *src,
 			FbCompositeOperand   *msk,
 			FbCompositeOperand   *dst);
 
-void
-fbCombineDisjointOverReverseU (FbCompositeOperand    *src,
-			       FbCompositeOperand    *msk,
-			       FbCompositeOperand    *dst);
-
-void
-fbCombineDisjointOverReverseC (FbCompositeOperand    *src,
-			       FbCompositeOperand    *msk,
-			       FbCompositeOperand    *dst);
+#define fbCombineDisjointOverReverseU fbCombineSaturateU
+#define fbCombineDisjointOverReverseC fbCombineSaturateC
 
 void
 fbCombineDisjointInU (FbCompositeOperand	    *src,