RFC: meta-window-actor: Add get_image() function

This adds a meta_window_actor_get_image function to MetaWindowActor which returns a ClutterContent of the new type ClutterImageTexture (basically a simple CoglTexture wrapper). When possible, is just uses the underlying CoglTexture, creating a subtexture from it. If that's not possible (eglstream, surface transforms etc) or the the MetaWindowActor has child actors (subsurfaces), it draws to a new CoglTexture accordingly.

This allows us to make window screenshots and animations (and screencasting, not yet implemented) of wayland clients with subsurfaces and should be a small performance improvement for respective window animations.

Required for gnome-shell!341 (closed)

Fixes #146 (closed)