xlib: Aquire display before using it in DEBUG message.
gl: Separate framebuffer bind from destination selection

Disentangle the action of binding the framebuffer from setting the
destination. This straightens up the code a bit and avoids some redundant
operations (such as reacquiring the context) when simply switching from
the multi-sample framebuffer to the single-sample framebuffer and vice
versa.
gl: Update transformation when surface size changes

In my previous commit I mistakenly removed the transformation matrix
update when cairo_gl_surface_set_size is called. This change restores
it.