Add XferProcessor factory in GrPaint and GrDrawState.

include/gpu/GrXferProcessor.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrXferProcessor_DEFINED
+#define GrXferProcessor_DEFINED
+
+#include "GrColor.h"
+#include "GrFragmentProcessor.h"
+#include "GrTypes.h"
+#include "SkXfermode.h"
+
+/**
+ * GrXferProcessor is responsible for implementing the xfer mode that blends the src color and dst
+ * color. It does this by emitting fragment shader code and controlling the fixed-function blend
+ * state. The inputs to its shader code are the final computed src color and fractional pixel
+ * coverage. The GrXferProcessor's shader code writes the fragment shader output color that goes
+ * into the fixed-function blend. When dual-source blending is available, it may also write a
+ * seconday fragment shader output color. When allowed by the backend API, the GrXferProcessor may
+ * read the destination color. The GrXferProcessor is responsible for setting the blend coefficients
+ * and blend constant color.
+ *
+ * A GrXferProcessor is never installed directly into our draw state, but instead is created from a
+ * GrXPFactory once we have finalized the state of our draw.
+ */
+class GrXferProcessor : public GrFragmentProcessor {
+private:
+
+    typedef GrFragmentProcessor INHERITED;
+};
+
+/**
+ * We install a GrXPFactory (XPF) early on in the pipeline before all the final draw information is
+ * known (e.g. whether there is fractional pixel coverage, will coverage be 1 or 4 channel, is the
+ * draw opaque, etc.). Once the state of the draw is finalized, we use the XPF along with all the
+ * draw information to create a GrXferProcessor (XP) which can implement the desired blending for
+ * the draw.
+ *
+ * Before the XP is created, the XPF is able to answer queries about what functionality the XPs it
+ * creates will have. For example, can it create an XP that supports RGB coverage or will the XP
+ * blend with the destination color.
+ */
+class GrXPFactory : public GrProgramElement {
+public:
+    virtual const GrXferProcessor* createXferProcessor() const = 0;
+
+    /**
+     * This function returns true if the GrXferProcessor generated from this factory will be able to
+     * correctly blend when using RGB coverage. The knownColor and knownColorFlags represent the
+     * final computed color from the color stages.
+     */
+    virtual bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const = 0;
+
+private:
+    typedef GrProgramElement INHERITED;
+};
+
+#endif
+