ui: refactor using a common qemu_pixman_shareable

ui: refactor using a common qemu_pixman_shareable

Use a common shareable type for win32 & unix, and helper functions.
This simplify the code as it avoids a lot of #ifdef'ery.

Note: if it helps review, commits could be reordered to introduce the
common type before introducing shareable memory for unix.

Suggested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20241008125028.1177932-19-marcandre.lureau@redhat.com>