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

src/core/SkCanvas.cpp

Index: src/core/SkCanvas.cpp
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index 5ba6e8f6dc399c7c71fa8c858425ba826ddb6378..bd3c2abbc1f8a28fa36fe66b6f1de88bc44971fb 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -2305,3 +2305,18 @@ int SkCanvas::LayerIter::y() const { return fImpl->getY(); }
 ///////////////////////////////////////////////////////////////////////////////
 
 SkCanvas::ClipVisitor::~ClipVisitor() { }
+
+///////////////////////////////////////////////////////////////////////////////
+
+SkCanvas* SkCanvas::NewRaster(const SkImageInfo& info) {
+    SkBitmap bitmap;
+    if (!bitmap.allocPixels(info)) {
+        return NULL;
+    }
+
+    // should this functionality be moved into allocPixels()?
+    if (kOpaque_SkAlphaType != info.alphaType()) {
+        bitmap.eraseColor(0);

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

Same.

+    }
+    return SkNEW_ARGS(SkCanvas, (bitmap));
+}