Implement `alpha-background' on PGTK

Implement `alpha-background' on PGTK

* src/ftcrfont.c (ftcrfont_draw): Respect alpha-background where
appropriate.
* src/gtkutil.c (xg_create_frame_widgets): Swap some
conditionals around to get the right visual and signal handlers
on PGTK.
(xg_widget_style_updated): Likewise.
* src/pgtkfns.c (pgtk_set_alpha_background): New function.
(pgtk_frame_parm_handlers): Add it instead of the generic param
handler.
(Fx_create_frame): Fix typo.
* src/pgtkterm.c (pgtk_fill_rectangle):
(pgtk_set_cr_source_with_gc_foreground):
(pgtk_set_cr_source_with_gc_background):
(pgtk_set_cr_source_with_color): New argument
`respect_alpha_backend'.  All callers changed.
* src/pgtkterm.h: Update function prototypes.