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

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index bd66f7ca849644daa0e42b69af29000f1b0238ad..f363a3a5d83276136dcf975c89bda2abdd61b395 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -71,6 +71,7 @@ void SkLinearGradient::flatten(SkWriteBuffer& buffer) const {
     buffer.writePoint(fEnd);
 }
 
+#if 0
 bool SkLinearGradient::setContext(const SkBitmap& device, const SkPaint& paint,
                                  const SkMatrix& matrix) {
     if (!this->INHERITED::setContext(device, paint, matrix)) {
@@ -89,6 +90,7 @@ bool SkLinearGradient::setContext(const SkBitmap& device, const SkPaint& paint,
     }
     return true;
 }
+#endif
 
 #define NO_CHECK_ITER               \
     do {                            \
@@ -196,6 +198,7 @@ void shadeSpan_linear_repeat(TileProc proc, SkFixed dx, SkFixed fx,
 
 }
 
+#if 0
 void SkLinearGradient::shadeSpan(int x, int y, SkPMColor* SK_RESTRICT dstC,
                                 int count) {
     SkASSERT(count > 0);
@@ -244,6 +247,7 @@ void SkLinearGradient::shadeSpan(int x, int y, SkPMColor* SK_RESTRICT dstC,
         } while (--count != 0);
     }
 }
+#endif
 
 SkShader::BitmapType SkLinearGradient::asABitmap(SkBitmap* bitmap,
                                                 SkMatrix* matrix,
@@ -377,6 +381,7 @@ void shadeSpan16_linear_repeat(TileProc proc, SkFixed dx, SkFixed fx,
 }
 }
 
+#if 0
 static bool fixed_nearly_zero(SkFixed x) {
     return SkAbs32(x) < (SK_Fixed1 >> 12);
 }
@@ -432,6 +437,7 @@ void SkLinearGradient::shadeSpan16(int x, int y,
         } while (--count != 0);
     }
 }
+#endif
 
 #if SK_SUPPORT_GPU