Revert of remove (dumb) canvas::NewRaster, and rename surface::NewRasterPMColor to N32Premul

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 46dc3774f0b2f6c6e1d987de8493f700bf351955..ff60c57fa7ec325a7678fbb8e1e28629b83e71a2 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -60,6 +60,35 @@
 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 caller can access the pixels after drawing into this canvas by
+     *  calling readPixels() or peekPixels().
+     *
+     *  If the requested ImageInfo is opaque (either the colortype is
+     *  intrinsically opaque like RGB_565, or the info's alphatype is kOpaque)
+     *  then the pixel memory may be uninitialized. Otherwise, the pixel memory
+     *  will be initialized to 0, which is interpreted as transparent.
+     *
+     *  On failure, return NULL. This can fail for several reasons:
+     *  1. the memory allocation failed (e.g. request is too large)
+     *  2. invalid ImageInfo (e.g. negative dimensions)
+     *  3. unsupported ImageInfo for a canvas
+     *      - kUnknown_SkColorType, kIndex_8_SkColorType
+     *      - kIgnore_SkAlphaType
+     *      - this list is not complete, so others may also be unsupported
+     *
+     *  Note: it is valid to request a supported ImageInfo, but with zero
+     *  dimensions.
+     */
+    static SkCanvas* NewRaster(const SkImageInfo&);
+
+    static SkCanvas* NewRasterN32(int width, int height) {
+        return NewRaster(SkImageInfo::MakeN32Premul(width, height));
+    }
 
     /**
      *  Attempt to allocate raster canvas, matching the ImageInfo, that will draw directly into the