windowManager: Use an off-screen buffer for window dimming

windowManager: Use an off-screen buffer for window dimming

The way the window dimmer shader is applied will cause rendering errors with
the rounded corners, invisible borders or shaped textures since it doesn't deal
well with the multitexturing used by the MetaShapedTexture. Use an off-screen
buffer to flatten the texture before being applied.

https://bugzilla.gnome.org/show_bug.cgi?id=659302