b=568526 (related); add env var for switching 2d canvas to image surfaces; r=bz

content/canvas/src/nsCanvasRenderingContext2D.cpp

@@ -46,6 +46,7 @@
 #endif
 
 #include "prmem.h"
+#include "prenv.h"
 
 #include "nsIServiceManager.h"
 
@@ -950,8 +951,12 @@ nsCanvasRenderingContext2D::SetDimensions(PRInt32 width, PRInt32 height)
         if (mOpaque)
             format = gfxASurface::ImageFormatRGB24;
 
-        surface = gfxPlatform::GetPlatform()->CreateOffscreenSurface
-            (gfxIntSize(width, height), format);
+        if (PR_GetEnv("MOZ_CANVAS_IMAGE_SURFACE")) {
+            surface = new gfxImageSurface(gfxIntSize(width, height), format);
+        } else {
+            surface = gfxPlatform::GetPlatform()->CreateOffscreenSurface
+                (gfxIntSize(width, height), format);
+        }
 
         if (surface->CairoStatus() != 0) {
           surface = NULL;