Implement Porter Duff XP with a blend table

Removes the runtime logic used by PorterDuffXferProcessor to decide
blend coeffs and shader outputs, and instead uses a compile-time
constant table of pre-selected blend formulas. Separates out the dst
read fallback into its own XP.

Introduces a new blend strategy for srcCoeff=0 that can apply coverage
with a reverse subtract blend equation instead of dual source
blending.

Adds new macros in GrBlend.h to analyze blend formulas both runtime.

Removes kSetCoverageDrawing_OptFlag and GrSimplifyBlend as they are no
longer used.

Adds a GM that verifies all xfermodes, including arithmetic, with the
color/coverage invariants used by Porter Duff.

Adds a unit test that verifies each Porter Duff formula with every
color/coverage invariant.

Major changes:

 * Uses a reverse subtract blend equation for coverage when srcCoeff=0
   (clear, dst-out [Sa=1], dst-in, modulate). Platforms that don't
   support dual source blending no longer require a dst copy for
   dst-in and modulate.

 * Sets BlendInfo::fWriteColor to false when the blend does not modify
   the dst. GrGLGpu will now use glColorMask instead of blending for
   these modes (dst, dst-in [Sa=1], modulate ignored for [Sc=1]).

 * Converts all SA blend coeffs to One for opaque inputs, and ISA to
   Zero if there is also no coverage. (We keep ISA around when there
   is coverage because we use it to tweak alpha for coverage.)

 * Abandons solid white optimizations for the sake of simplicity
   (screen was the only mode that previous had solid white opts).

Minor differences:

 * Inconsequential differences in opt flags (e.g. we now return
   kCanTweakAlphaForCoverage_OptFlag even when there is no coverage).

 * Src coeffs when the shader outputs 0.

 * IS2C vs IS2A when the secondary output is scalar.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/9a70920db22b6309c671f8e5d519bb95570e4414

Review URL: https://codereview.chromium.org/1124373002

1 files changed, 5 insertions, 4 deletions

diff --git a/include/gpu/effects/GrPorterDuffXferProcessor.h b/include/gpu/effects/GrPorterDuffXferProcessor.h
index f8fc1a38cc..e4f72203f6 100644
--- a/include/gpu/effects/GrPorterDuffXferProcessor.h
+++ b/include/gpu/effects/GrPorterDuffXferProcessor.h
@@ -24,7 +24,7 @@ public:
                             GrXPFactory::InvariantOutput*) const override;
 
 private:
-    GrPorterDuffXPFactory(GrBlendCoeff src, GrBlendCoeff dst); 
+    GrPorterDuffXPFactory(SkXfermode::Mode);
 
     GrXferProcessor* onCreateXferProcessor(const GrCaps& caps,
                                            const GrProcOptInfo& colorPOI,
@@ -37,14 +37,15 @@ private:
 
     bool onIsEqual(const GrXPFactory& xpfBase) const override {
         const GrPorterDuffXPFactory& xpf = xpfBase.cast<GrPorterDuffXPFactory>();
-        return (fSrcCoeff == xpf.fSrcCoeff && fDstCoeff == xpf.fDstCoeff);
+        return fXfermode == xpf.fXfermode;
     }
 
     GR_DECLARE_XP_FACTORY_TEST;
+    static void TestGetXPOutputTypes(const GrXferProcessor*, int* outPrimary, int* outSecondary);
 
-    GrBlendCoeff fSrcCoeff;
-    GrBlendCoeff fDstCoeff;
+    SkXfermode::Mode fXfermode;
 
+    friend class GrPorterDuffTest; // for TestGetXPOutputTypes()
     typedef GrXPFactory INHERITED;
 };