effects/screenshot: Port away from window quads

For the scene redesign, window quads need to be removed from the effects api. This change ports the screenshot effect away from quads.

In order to exclude the server-side decoration from the window screenshot, the effect will render the contents of the client geometry into the offscreen texture. If the window is client-side decorated, the entire window will be rendered as before.

The good thing about this approach is that the screenshot effect will do less work, it won't loop over window quads to compute the bounding rect or filter out the deco quads.

cc #30