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

include/core/SkCanvas.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCanvas_DEFINED
 #define SkCanvas_DEFINED
 
@@ skipping 31 matching lines @@
     of the object being drawn is held by the Paint, which is provided as a
     parameter to each of the draw() methods. The Paint holds attributes such as
     color, typeface, textSize, strokeWidth, shader (e.g. gradients, patterns),
     etc.
 */
 class SK_API SkCanvas : public SkRefCnt {
 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.
      */
     SkCanvas();
 
     /**
      *  Creates a canvas of the specified dimensions, but explicitly not backed
      *  by any device/pixels. Typically this use used by subclasses who handle
      *  the draw calls in some other way.
      */
@@ skipping 1249 matching lines @@
     bool asROBitmap(SkBitmap*) const;
 
 private:
     SkBitmap    fBitmap;    // used if peekPixels() fails
     const void* fAddr;      // NULL on failure
     SkImageInfo fInfo;
     size_t      fRowBytes;
 };
 
 #endif