add NewRasterDirect for creating a canvas drawing to pre-allocated pixels

src/core/SkCanvas.cpp

Index: src/core/SkCanvas.cpp
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index 4ac7b29826e790d0d45f2eaecb3a794856a14d5f..e13769a20af7eb08107b2cd5fe9cd1c0eebd49e9 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -2549,3 +2549,22 @@ SkCanvas* SkCanvas::NewRaster(const SkImageInfo& info) {
     }
     return SkNEW_ARGS(SkCanvas, (bitmap));
 }
+
+SkCanvas* SkCanvas::NewRasterDirect(const SkImageInfo& info, void* pixels, size_t rowBytes) {
+    if (!supported_for_raster_canvas(info)) {
+        return NULL;
+    }
+    
+    SkBitmap bitmap;
+    if (!bitmap.installPixels(info, pixels, rowBytes)) {
+        return NULL;
+    }
+    
+    // should this functionality be moved into allocPixels()?
+    if (!bitmap.info().isOpaque()) {
+        bitmap.eraseColor(0);
+    }
+    return SkNEW_ARGS(SkCanvas, (bitmap));
+}
+
+