[CSS Blending] Implement mix-blend-mode in software.

Source/core/rendering/CompositingReasons.h

Index: Source/core/rendering/CompositingReasons.h
diff --git a/Source/core/rendering/CompositingReasons.h b/Source/core/rendering/CompositingReasons.h
index 47713ee962bdf0c407679259c5e055ea4f036b2f..af37e11c9698155e7abd0495192b6a285afbeb00 100644
--- a/Source/core/rendering/CompositingReasons.h
+++ b/Source/core/rendering/CompositingReasons.h
@@ -24,6 +24,8 @@ const uint64_t CompositingReasonFilters                                = UINT64_
 const uint64_t CompositingReasonPositionFixed                          = UINT64_C(1) << 8;
 const uint64_t CompositingReasonPositionSticky                         = UINT64_C(1) << 9;
 const uint64_t CompositingReasonOverflowScrollingTouch                 = UINT64_C(1) << 10;
+
+// FIXME: remove the following compositing reason since it's no longer used.

[shawnsingh, 2013/09/10 21:23:40]

I think it's best to address this FIXME in this patch.   It should be possible
to remove it on the chromium side first to make this possible.

[mitica, 2013/09/11 16:04:14]

The only problem with this would be that removing the blending flag from
https://code.google.com/p/chromium/codesearch#chromium/src/cc/layers/composit...
would create a 1 bit gap in the reasons list.
This cannot be avoided due to the compile asserts in blink in
https://code.google.com/p/chromium/codesearch#chromium/src/webkit/renderer/co...

Anyhow, could you please have a look at https://codereview.chromium.org/23530026
first? After this is finished, I will be able to completely remove the blending
compositing reason on the blink side :)

 const uint64_t CompositingReasonBlending                               = UINT64_C(1) << 11;
 
 // Overlap reasons that require knowing what's behind you in paint-order before knowing the answer