Bug 444690. gfxPlatformGTK::CreateOffscreenSurface should clear the surface to be consistent with other platforms. r=vlad

gfx/thebes/src/gfxPlatformGtk.cpp

@@ -43,6 +43,7 @@
 
 #include "gfxFontconfigUtils.h"
 #include "gfxPangoFonts.h"
+#include "gfxContext.h"
 
 #include "cairo.h"
 #include <gtk/gtk.h>
@@ -207,6 +208,12 @@ gfxPlatformGtk::CreateOffscreenSurface(const gfxIntSize& size,
 #endif
     }
 
+    if (newSurface) {
+        gfxContext tmpCtx(newSurface);
+        tmpCtx.SetOperator(gfxContext::OPERATOR_CLEAR);
+        tmpCtx.Paint();
+    }
+
     return newSurface.forget();
 }