Make pixmap exportable in ms_do_pageflip

Otherwise glamor_gbm_bo_from_pixmap will fail and cause the pageflip
to fail.

Fixes #68

Signed-off-by: Yuxuan Shui <yshui@hadean.com>
3 jobs for master in 5 minutes and 22 seconds (queued for 1 minute and 30 seconds)
Status Job ID Name Coverage
  Docker Image
passed debian-testing #144992

00:00:27

 
  Build And Test
passed autotools-build-and-test #144993

00:04:44

passed meson-build-and-test #144994

00:00:56