Add XP to handle the cases where we disable color write.

include/gpu/GrXferProcessor.h

Index: include/gpu/GrXferProcessor.h
diff --git a/include/gpu/GrXferProcessor.h b/include/gpu/GrXferProcessor.h
index 98585c6a5374b311ef532b352567c158a26977eb..2c9961fc6b625cc231fd001aa1f9270f00910e69 100644
--- a/include/gpu/GrXferProcessor.h
+++ b/include/gpu/GrXferProcessor.h
@@ -86,18 +86,19 @@ public:
      * A caller who calls this function on a XP is required to honor the returned OptFlags
      * and color values for its draw.
      */
-    // TODO: remove need for colorWriteDisabled once colorWriteDisabled is its own XP.
     virtual OptFlags getOptimizations(const GrProcOptInfo& colorPOI,
                                       const GrProcOptInfo& coveragePOI,
-                                      bool colorWriteDisabled,
                                       bool doesStencilWrite,
                                       GrColor* overrideColor,
                                       const GrDrawTargetCaps& caps) = 0;
 
     struct BlendInfo {
+        BlendInfo() : fWriteColor(true) {}
+
         GrBlendCoeff fSrcBlend;
         GrBlendCoeff fDstBlend;
         GrColor      fBlendConstant;
+        bool         fWriteColor;
     };
 
     virtual void getBlendInfo(BlendInfo* blendInfo) const = 0;
@@ -176,9 +177,8 @@ public:
      * certain state information (colorWriteDisabled) to determine whether
      * coverage can be handled correctly.
      */
-    // TODO: remove need for colorWriteDisabled once colorWriteDisabled is its own XP.
-    virtual bool canApplyCoverage(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
-                                  bool colorWriteDisabled) const = 0;
+    virtual bool canApplyCoverage(const GrProcOptInfo& colorPOI,

[bsalomon, 2014/12/19 16:40:34]

does it fit on one line? :)

[egdaniel, 2014/12/19 19:53:45]

Nope

+                                  const GrProcOptInfo& coveragePOI) const = 0;
 
 
     struct InvariantOutput {
@@ -192,9 +192,8 @@ public:
      * this xp factory. The invariant color information returned by this function refers to the
      * final color produced after all blending.
      */
-    // TODO: remove need for colorWriteDisabled once only XP can read dst.
     virtual void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
-                                    bool colorWriteDisabled, InvariantOutput*) const = 0;
+                                    InvariantOutput*) const = 0;
 
     /**
      * Determines whether multiplying the computed per-pixel color by the pixel's fractional
@@ -203,6 +202,14 @@ public:
      */
     virtual bool canTweakAlphaForCoverage() const = 0;
 
+    /**
+     *  Returns true if the XP generated by this factory will read dst.
+     */
+    // TODO: Currently this function must also check if the color/coverage stages read dst.
+    //       Once only XP's can read dst we can remove the ProcOptInfo's from this function.
+    virtual bool willReadDst(const GrProcOptInfo& colorPOI,

[bsalomon, 2014/12/19 16:40:34]

At first I thought this was a reintroduction of the recently departed
willBlendWithDst. This could be confusing since this class is all tied up with
blending src and dst colors. Maybe it should be requiresDstInFragmentShader? 

[egdaniel, 2014/12/19 19:53:45]

So the fact that it is even checking Fragment shaders right now is a side effect
of read dst not being xp only. And it also is checking the XP itself, but right
now all the XP's we have would simply be adding "|| false" to the checks. In the
future this function should really just be an inline function that just returns
false or true with no logic at all.

+                             const GrProcOptInfo& coveragePOI) const = 0;
+
     bool isEqual(const GrXPFactory& that) const {
         if (this->classID() != that.classID()) {
             return false;