dri3: Always flush glamor before sharing pixmap storage with clients

Even if glamor_gbm_bo_from_pixmap / glamor_fd_from_pixmap themselves
don't trigger any drawing, there could already be unflushed drawing to
the pixmap whose storage we share with a client.
12 jobs for dri3_fd_from_pixmap-glamor-always-flush in 1 minute and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #270993
debian-testing

00:00:29

 
  Build
passed #270996
xserver-1.13

00:00:38

passed #270997
xserver-1.14

00:00:31

passed #270998
xserver-1.15

00:00:38

passed #270999
xserver-1.16

00:00:39

passed #271000
xserver-1.17

00:00:38

passed #271001
xserver-1.18

00:00:35

passed #271002
xserver-1.18-clang

00:00:43

passed #271003
xserver-1.19

00:00:35

passed #271004
xserver-1.19-clang

00:00:37

passed #270994
xserver-1.20

00:00:46

passed #270995
xserver-1.20-clang

00:00:40