Add convex polygon rendering effect and GM to test it.

src/gpu/effects/GrConvexPolyEffect.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 GrConvexPolyEffect_DEFINED
+#define GrConvexPolyEffect_DEFINED
+
+#include "GrDrawTargetCaps.h"
+#include "GrEffect.h"
+#include "GrVertexEffect.h"
+
+class GrGLConvexPolyEffect;
+class SkPath;
+
+/**
+ * An effect that renders a convex polygon. It is intended to be used as a coverage effect.
+ * Bounding geometry is rendered and the effect computes coverage based on the fragment's
+ * position relative to the polygon.
+ */
+class GrConvexPolyEffect : public GrEffect {
+public:
+    /** This could be expanded to include a AA hairline mode. If so, unify with GrBezierEffect's
+        enum. */
+    enum EdgeType {
+        kFillNoAA_EdgeType,
+        kFillAA_EdgeType,
+        
+        kLastEdgeType = kFillAA_EdgeType,
+    };
+
+    enum {
+        kEdgeTypeCnt = kLastEdgeType + 1,
+        kMaxEdges = 8,
+    };
+
+    /**
+     * edges is a set of n edge equations where n is limited to kMaxEdges. It contains 3*n values.
+     * The edges should form a convex polygon. The positive half-plane is considered to be the
+     * inside. The equations should be normalized such that the first two coefficients are a unit
+     * 2d vector.
+     *
+     * Currently the edges are specified in device space. In the future we may prefer to specify
+     * them in src space. There are a number of ways this could be accomplished but we'd probably
+     * have to modify the effect/shaderbuilder interface to make it possible (e.g. give access
+     * to the view matrix or untransformed positions in the fragment shader).
+     */
+    static GrEffectRef* Create(EdgeType edgeType, int n, const SkScalar edges[]) {
+        if (n <= 0 || n > kMaxEdges) {
+            return NULL;
+        }
+        return CreateEffectRef(AutoEffectUnref(SkNEW_ARGS(GrConvexPolyEffect,
+                                                          (edgeType, n, edges))));
+    }
+
+    /**
+     * Creates an effect that clips against the path. If the path is not a convex polygon, is
+     * inverse filled, or has too many edges, this will return NULL.
+     */
+    static GrEffectRef* Create(EdgeType, const SkPath&);
+
+    virtual ~GrConvexPolyEffect();
+
+    static const char* Name() { return "ConvexPoly"; }
+
+    EdgeType getEdgeType() const { return fEdgeType; }
+
+    int getEdgeCount() const { return fEdgeCount; }
+
+    const SkScalar* getEdges() const { return fEdges; }
+
+    typedef GrGLConvexPolyEffect GLEffect;
+
+    virtual void getConstantColorComponents(GrColor* color, uint32_t* validFlags) const SK_OVERRIDE;
+
+    virtual const GrBackendEffectFactory& getFactory() const SK_OVERRIDE;
+
+private:
+    GrConvexPolyEffect(EdgeType edgeType, int n, const SkScalar edges[]);
+
+    virtual bool onIsEqual(const GrEffect& other) const SK_OVERRIDE;
+
+    EdgeType fEdgeType;
+    int      fEdgeCount;
+    SkScalar fEdges[3 * kMaxEdges];
+
+    GR_DECLARE_EFFECT_TEST;
+
+    typedef GrEffect INHERITED;
+};
+
+
+#endif