[cairo] [win32] crash in _cairo_surface_paint

[Maximilian Heinzler]

With the latest git, I recognized that cairo_show_text/glyphs crash in
_cairo_surface_paint when using win32 fonts on an image surface, because
it tries to call the backend's paint function, which does not exist in
the win32 backend.

I think the bug is in some function called from cairo_show_text/glyphs,
but only for image surfaces, e. g. not for win32 surfaces.