eglnative: Add clutter-stage-window implementation

Add a ClutterStageEglNative implemennation that implements
can_clip_redraws so that clipped redraws can work
on eglnative.

https://bugzilla.gnome.org/show_bug.cgi?id=726341
clutter-stage-wayland: Enable clipped redraws

_clutter_stage_window_can_clip_redraws is used to check for clipped redraws
support but can_clip_redraws is not implemented by clutter-stage-wayland so
it always returns FALSE causing full screen redraws.

Fix that by implementing can_clip_redraws in clutter-stage-wayland.

https://bugzilla.gnome.org/show_bug.cgi?id=726315
stage-cogl: Fix feature check in clutter_stage_cogl_redraw

We do not strictly require the 'swap-region' Cogl feature in order to use
clipped redraws: they work equally well with just the 'buffer-age' Cogl
feature.

https://bugzilla.gnome.org/show_bug.cgi?id=726313