When a renderer switch happens, it is possible that when the surface
state is created, a buffer for the given surface is already available.
In that case, run the attach routine so that the pixel contents are
properly set. Otherwise, it would only be set when a new attach request
is made for that surface.

Also, change the drm backend so that it keeps the buffer reference in
the weston_surface when running with the pixman renderer. The pixman
renderer keeps a reference to it anyway, so it is never released
early.

This makes the renderer transition seamless, without leaving a black
screen as before.