Create fragment processor for performing input color blend with child processor

The new FP is used to implement SkXM::Mode color filters and SkXM::Mode image filters. Also, these now support all advanced SkXM::Mode xfermodes.

Review URL: https://codereview.chromium.org/1334293003
diff --git a/src/gpu/GrProcessor.cpp b/src/gpu/GrProcessor.cpp
index b3544b0..782b82b 100644
--- a/src/gpu/GrProcessor.cpp
+++ b/src/gpu/GrProcessor.cpp
@@ -50,7 +50,7 @@
  * we verify the count is as expected.  If a new factory is added, then these numbers must be
  * manually adjusted.
  */
-static const int kFPFactoryCount = 40;
+static const int kFPFactoryCount = 39;
 static const int kGPFactoryCount = 14;
 static const int kXPFactoryCount = 5;