compositor: call configure on surfaces with a null buffer too

compositor: call configure on surfaces with a null buffer too

This way the shell can know when a surface has been unmapped by
checking the value returned by weston_surface_is_mapped(surface).
The configure handlers have now width and height parameters, so
they do not need anymore to check manually the buffer size.
If a surface's buffer is NULL the width and height passed to the
configure are both 0.
Configure is now only called after an attach. The variable
weston_surface.pending.newly_attached is set to 1 on attach, and
after the configure call is reset to 0.