add peekPixels to canvas

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 7235efb2f0a9cd3b23132541d9c695b0c81e1704..cc1bfca612277a00c5113798b9af1a00e2ae3d63 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -126,6 +126,18 @@ public:
     ///////////////////////////////////////////////////////////////////////////
 
     /**
+     *  If the canvas has direct access to its pixels (i.e. they are in local
+     *  RAM) return the const-address of those pixels, and if not null, return

[hal.canary, 2014/02/12 15:11:02]

And if the canvas has a concept of "pixels" at all.

[reed1, 2014/02/12 17:24:10]

Done.

+     *  the ImageInfo and rowBytes. The returned address is only valid while
+     *  the canvas object is in scope and unchanged. Any API calls made on the
+     *  canvas can invalidate this address.
+     *
+     *  On failure, returns NULL and the info and rowBytes parameters are
+     *  ignored.
+     */
+    const void* peekPixels(SkImageInfo* info, size_t* rowBytes);
+
+    /**
      * This enum can be used with read/writePixels to perform a pixel ops to or
      * from an 8888 config other than Skia's native config (SkPMColor). There
      * are three byte orders supported: native, BGRA, and RGBA. Each has a
@@ -1019,6 +1031,9 @@ protected:
     // default impl defers to getDevice()->newSurface(info)
     virtual SkSurface* onNewSurface(const SkImageInfo&);
 
+    // default impl defers to its device
+    virtual const void* onPeekPixels(SkImageInfo*, size_t* rowBytes);
+
     // Returns the canvas to be used by DrawIter. Default implementation
     // returns this. Subclasses that encapsulate an indirect canvas may
     // need to overload this method. The impl must keep track of this, as it