weston: Add weston_get_backend_type from Weston 14

weston: Add weston_get_backend_type from Weston 14

This helper is from Weston 14 and helps identify the outputs created by
other remote back-ends (like PipeWire/RDP/VNC) which we can further use to
further differentiate between different types of back- ends.

Bug-AGL: SPEC-5324
Change-Id: Ie7537380eaa81a4c48d4ef5c09598ab102310842
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/30680
Tested-by: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>