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"
+
+class GrDrawState;
+
+/**
+ * A GrXferProcessor is used to combine the output of the color and coverage stages into one or two

[bsalomon, 2014/12/01 19:24:39]

I think this comment will be confusing to someone who is extending Skia and not
familiar with Ganesh's internals. Here is my attempt at rewriting a subset of
the comment:

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 secondary 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.

[egdaniel, 2014/12/02 15:07:09]

Done.

+ * output colors (two if dual source is enabled). All blending is handled internally inside the xfer
+ * processor and thus the client code can be oblivious to how the blending works. A GrXP is never
+ * created directly by a client. Instead, a GrXPFactory class is used. At the time of snapping the
+ * draw state and creating optimizations, the XPFactory will create an XP. This XP also serves as a
+ * feedback loop into the OptDrawState, informing the OptDrawState what additional optimizations to
+ * perform based on blending. The XP does not have local coords and will never read the fragment
+ * position. However, it can possibly read the destination color when doing blending. The final draw
+ * state will always have one and only one GrXferProcessor.
+ */
+class GrXferProcessor : public GrFragmentProcessor {
+private:
+    virtual bool onIsEqual(const GrFragmentProcessor&) const = 0;

[bsalomon, 2014/12/01 19:24:40]

pure virtual override?

[egdaniel, 2014/12/02 15:07:09]

because we all love code that does nothing of course

+
+    typedef GrFragmentProcessor INHERITED;
+};
+
+class GrXPFactory : public GrProgramElement {
+public:
+    virtual const GrXferProcessor* createXferProcessor() const = 0;
+
+    virtual bool supportsRGBCoverage(const GrDrawState&) const { return false; }

[bsalomon, 2014/12/01 19:24:40]

comment about what this means?

Also, GrDrawState is not public so can't appear in a public header. I realize it
is forward declared, but an implementer of this function outside of src/ is not
allowed to know its definition. 

[egdaniel, 2014/12/02 15:07:09]

Done.

+
+private:
+    typedef GrProgramElement INHERITED;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+
+/**
+ * This creates a factory outside of the effect memory pool. The factory's destructor will be called
+ * at global destruction time. NAME will be the name of the created GrXPFactory.
+ */
+#define GR_CREATE_STATIC_XP_FACTORY(NAME, XPF_CLASS, ARGS)                                     \
+static SkAlignedSStorage<sizeof(XPF_CLASS)> g_##NAME##_Storage;                                \
+static GrXPFactory* NAME SkNEW_PLACEMENT_ARGS(g_##NAME##_Storage.get(), XPF_CLASS, ARGS);      \
+static SkAutoTDestroy<GrXPFactory> NAME##_ad(NAME);
+
+#endif
+