Fix UB function problems for shaders and mask.

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 34cb648022ca25593943ba054f7d196ad2c26bc2..9b48697d20ae4e673951dfb351cba7a714782abb 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -149,7 +149,11 @@ public:
          */
         virtual void shadeSpan(int x, int y, SkPMColor[], int count) = 0;
 
-        typedef void (*ShadeProc)(void* ctx, int x, int y, SkPMColor[], int count);
+        /**
+         * The const void* ctx is only const because all the implementations are const.
+         * This can be changed to non-const if a new shade proc needs to change the ctx.
+         */
+        typedef void (*ShadeProc)(const void* ctx, int x, int y, SkPMColor[], int count);
         virtual ShadeProc asAShadeProc(void** ctx);
 
         /**