Implement support for a Context parameter in image filters

Some upcoming work (support for expanding crop rects) requires
the clip bounds to be available during filter traversal. This change
replaces the SkMatrix parameter in the onFilterImage() traversals
with a Context parameter. It contains the CTM, as well as the clip
bounds.

BUG=skia:
R=reed@google.com

Review URL: https://codereview.chromium.org/189913021

git-svn-id: http://skia.googlecode.com/svn/trunk@13803 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/effects/SkOffsetImageFilter.h b/include/effects/SkOffsetImageFilter.h
index 93fc273..8d9db64 100644
--- a/include/effects/SkOffsetImageFilter.h
+++ b/include/effects/SkOffsetImageFilter.h
@@ -26,7 +26,7 @@
     SkOffsetImageFilter(SkReadBuffer& buffer);
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
 
-    virtual bool onFilterImage(Proxy*, const SkBitmap& src, const SkMatrix&,
+    virtual bool onFilterImage(Proxy*, const SkBitmap& src, const Context&,
                                SkBitmap* result, SkIPoint* loc) const SK_OVERRIDE;
     virtual bool onFilterBounds(const SkIRect&, const SkMatrix&, SkIRect*) const SK_OVERRIDE;