Fix filter primitive bounds computations.

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index 909a2f82840e93c2c6842a3407ef3b67ec3eb893..e9f856be209949f4153aa4436f0e6cba121acae0 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -156,6 +156,12 @@ public:
      */
     bool filterBounds(const SkIRect& src, const SkMatrix& ctm, SkIRect* dst) const;
 
+    Context mapContext(const Context& ctx) const {
+        SkIRect clipBounds;
+        this->onFilterNodeBounds(ctx.clipBounds(), ctx.ctm(), &clipBounds, MapDirection::kReverse_MapDirection);
+        return Context(ctx.ctm(), clipBounds, ctx.cache(), ctx.sizeConstraint());
+    }
+
     /**
      *  Returns true if the filter can be processed on the GPU.  This is most
      *  often used for multi-pass effects, where intermediate results must be
@@ -346,6 +352,13 @@ protected:
     // implementation recursively unions all input bounds, or returns false if
     // no inputs.
     virtual bool onFilterBounds(const SkIRect&, const SkMatrix&, SkIRect*) const;
+    enum MapDirection {
+        kForward_MapDirection,
+        kReverse_MapDirection
+    };
+
+    // Non-recursive version of the above.
+    virtual void onFilterNodeBounds(const SkIRect&, const SkMatrix&, SkIRect*, MapDirection direction) const;
 
     // Helper function which invokes filter processing on the input at the
     // specified "index". If the input is null, it leaves "result" and