wayland-shm: re-organize some of the swap/redraw logic

wayland-shm: re-organize some of the swap/redraw logic

Summary:
The way things were, we'd be committing a buffer we hadn't rendered to
yet.

Now redraw() contains all the attach/damage/commit logic, and swap() is
called afterwards to set up a new target buffer for the next render.

@fix

Reviewers: zmike, devilhorns

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D2875