Use gbm-format=argb8888 for primary output plane

Primary plane must support alpha for video "overlays" to work.
They're really underlays since they are below the primary plane,
and hence the need to have transparent regions in it.

BUG: 142883165
Change-Id: I5ed98747e0e1d5bae76c889af933e48b1326f0b8
diff --git a/debian/etc/xdg/weston/weston.ini b/debian/etc/xdg/weston/weston.ini
index e562dbd..573618b 100644
--- a/debian/etc/xdg/weston/weston.ini
+++ b/debian/etc/xdg/weston/weston.ini
@@ -1,4 +1,5 @@
 [core]
+gbm-format=argb8888
 shell=desktop-shell.so
 modules=systemd-notify.so
 xwayland=true