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.c

@@ -1001,7 +1001,6 @@ fbComposite (CARD8      op,
 	{
 	    switch (pSrc->format) {
 	    case PICT_a8r8g8b8:
-	    case PICT_x8r8g8b8:
 		switch (pDst->format) {
 		case PICT_a8r8g8b8:
 		case PICT_x8r8g8b8:
@@ -1016,7 +1015,6 @@ fbComposite (CARD8      op,
 		}
 		break;
 	    case PICT_a8b8g8r8:
-	    case PICT_x8b8g8r8:
 		switch (pDst->format) {
 		case PICT_a8b8g8r8:
 		case PICT_x8b8g8r8: