add SkCanvas::NewRaster factory -- a very common use-case in chrome

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index e9d4933d177112fed795e36746e3d482e414d4a8..80e5db26f83017fda33fbe1a0f1550a83976323c 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -49,6 +49,22 @@ public:
     SK_DECLARE_INST_COUNT(SkCanvas)
 
     /**
+     *  Attempt to allocate an offscreen raster canvas, matching the ImageInfo.
+     *  On success, return a new canvas that will draw into that offscreen.
+     *  The pixels will be uninitialized if the ImageInfo's alpha was kOpaque,
+     *  else they will be initialized to 0. The caller can access the pixels

[scroggo, 2014/02/27 19:16:06]

How about initializing them to SK_ColorTRANSPARENT?

+     *  after drawing into this canvas by calling readPixels() or peekPixels().
+     *
+     *  If the offscreen cannot be allocated, or the requested ImageInfo is not
+     *  supported, then NULL is returned.
+     */
+    static SkCanvas* NewRaster(const SkImageInfo&);
+
+    static SkCanvas* NewRasterN32(int width, int height) {
+        return NewRaster(SkImageInfo::MakeN32Premul(width, height));
+    }
+
+    /**
      *  Creates an empty canvas with no backing device/pixels, and zero
      *  dimensions.
      */