present: Drop flip_idler member from present_vblank_rec

It's redundant with the pixmap member of struct xwl_present_event.
5 jobs for present_wnmd-to-xwayland-present in 4 minutes and 41 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Docker Image
passed #10824572
debian-buster

00:00:28

 
  Build And Test
passed #10824573
autotools

00:04:12

passed #10824574
meson

00:02:53

passed #10824575
meson-noglamor

00:01:36

passed #10824576
mingw-cross-build

00:00:44