Add convex polygon rendering effect and GM to test it.

gm/beziereffects.cpp

Index: gm/beziereffects.cpp
diff --git a/gm/beziereffects.cpp b/gm/beziereffects.cpp
index c33674caad726da9a3cd30f044e8a418fc164a5d..87e2d0a21d85b94c5f3c420e252f4730d82ea2af 100644
--- a/gm/beziereffects.cpp
+++ b/gm/beziereffects.cpp
@@ -23,10 +23,12 @@
 
 // Position & KLM line eq values. These are the vertex attributes for Bezier curves. The last value
 // of the Vec4f is ignored.
+namespace {
 extern const GrVertexAttrib kAttribs[] = {
     {kVec2f_GrVertexAttribType, 0, kPosition_GrVertexAttribBinding},
     {kVec4f_GrVertexAttribType, sizeof(GrPoint), kEffect_GrVertexAttribBinding}
 };
+}
 
 static inline SkScalar eval_line(const SkPoint& p, const SkScalar lineEq[3], SkScalar sign) {
     return sign * (lineEq[0] * p.fX + lineEq[1] * p.fY + lineEq[2]);