Extract most of the mutable state of SkShader into a separate Context object.

src/effects/gradients/SkSweepGradient.cpp

Index: src/effects/gradients/SkSweepGradient.cpp
diff --git a/src/effects/gradients/SkSweepGradient.cpp b/src/effects/gradients/SkSweepGradient.cpp
index cb9d2763d5a3bed2af77d65a2be6c1ec198b6b83..e26a730204f09f11f7dcb3e55cbe59e5abd36ad6 100644
--- a/src/effects/gradients/SkSweepGradient.cpp
+++ b/src/effects/gradients/SkSweepGradient.cpp
@@ -52,6 +52,7 @@ void SkSweepGradient::flatten(SkWriteBuffer& buffer) const {
     buffer.writePoint(fCenter);
 }
 
+#if 0
 //  returns angle in a circle [0..2PI) -> [0..255]
 static unsigned SkATan2_255(float y, float x) {
     //    static const float g255Over2PI = 255 / (2 * SK_ScalarPI);
@@ -156,6 +157,7 @@ void SkSweepGradient::shadeSpan16(int x, int y, uint16_t* SK_RESTRICT dstC,
         }
     }
 }
+#endif
 
 /////////////////////////////////////////////////////////////////////