Bug 1028786, part 1 - Add a gfxUtils::ClearThebesSurface() helper method. r=mattwoodrow

--HG--
extra : rebase_source : 524ffe6d276ea9b51cf01d534f65c9b74b32b950

gfx/thebes/gfxUtils.cpp

@@ -4,6 +4,8 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "gfxUtils.h"
+
+#include "cairo.h"
 #include "gfxContext.h"
 #include "gfxImageSurface.h"
 #include "gfxPlatform.h"
@@ -993,6 +995,21 @@ gfxUtils::ConvertYCbCrToRGB(const PlanarYCbCrData& aData,
   }
 }
 
+/* static */ void gfxUtils::ClearThebesSurface(gfxASurface* aSurface)
+{
+  if (aSurface->CairoStatus()) {
+    return;
+  }
+  cairo_surface_t* surf = aSurface->CairoSurface();
+  if (cairo_surface_status(surf)) {
+    return;
+  }
+  cairo_t* ctx = cairo_create(surf);
+  cairo_set_operator(ctx, CAIRO_OPERATOR_CLEAR);
+  cairo_paint_with_alpha(ctx, 1.0);
+  cairo_destroy(ctx);
+}
+
 /* static */ TemporaryRef<DataSourceSurface>
 gfxUtils::CopySurfaceToDataSourceSurfaceWithFormat(SourceSurface* aSurface,
                                                    SurfaceFormat aFormat)

gfx/thebes/gfxUtils.h

@@ -13,6 +13,7 @@
 #include "mozilla/RefPtr.h"
 #include "nsPrintfCString.h"
 
+class gfxASurface;
 class gfxDrawable;
 class nsIntRegion;
 struct nsIntRect;
@@ -166,6 +167,11 @@ public:
                       unsigned char* aDestBuffer,
                       int32_t aStride);
 
+    /**
+     * Clears surface to transparent black.
+     */
+    static void ClearThebesSurface(gfxASurface* aSurface);
+
     /**
      * Creates a copy of aSurface, but having the SurfaceFormat aFormat.
      *