Avoid unnecessary window flushes

Avoid unnecessary window flushes

The code was calling _gdk_window_ref_cairo_surface in a few places
where the intent was not to read/write to the surface, but just look
at its type (to e.g. create a similar surface). This is bad, as that
operation causes a flush which may cause unnecessary work and/or
flashing. Instead we just get the impl surface in these cases.