Add a weston_buffer structure to replace wl_buffer

Add a weston_buffer structure to replace wl_buffer

This commit adds a weston_buffer structure to replace wl_buffer.  This way
we can hold onto buffers by just their resource.  In order to do this, the
every renderer.attach function has to fill in the weston_buffer.width and
weston_buffer.height fields.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>