Add all remaining separable xfer modes to GPU backend.

include/core/SkXfermode.h

Index: include/core/SkXfermode.h
===================================================================
--- include/core/SkXfermode.h	(revision 8762)
+++ include/core/SkXfermode.h	(working copy)
@@ -87,6 +87,9 @@
         [a, c] - Resulting (alpha, color) values
         For these equations, the colors are in premultiplied state.
         If no xfermode is specified, kSrcOver is assumed.
+        The modes are ordered by those that can be expressed as a pair of Coeffs, followed by those
+        that aren't Coeffs but have separable r,g,b computations, and finally
+        those that are not separable.
      */
     enum Mode {
         kClear_Mode,    //!< [0, 0]
@@ -107,9 +110,9 @@
         // Following blend modes are defined in the CSS Compositing standard:
         // https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#blending
         kScreen_Mode,
-        // all above modes can be expressed as pair of src/dst Coeffs
-        kCoeffModesCnt,
-        kOverlay_Mode = kCoeffModesCnt,
+        kLastCoeffMode = kScreen_Mode,
+
+        kOverlay_Mode,
         kDarken_Mode,
         kLighten_Mode,
         kColorDodge_Mode,
@@ -119,12 +122,12 @@
         kDifference_Mode,
         kExclusion_Mode,
         kMultiply_Mode,
+        kLastSeparableMode = kMultiply_Mode,
 
         kHue_Mode,
         kSaturation_Mode,
         kColor_Mode,
         kLuminosity_Mode,
-
         kLastMode = kLuminosity_Mode
     };