use a format that supports alpha directly

now we can unify the android and linux cases as well and drop the
BGRX support

Change-Id: I3c845913691d8194822423005d308cfa7ef13ec3
Reviewed-on: https://gerrit.libreoffice.org/20440Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>