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 debian-buster #10824572

00:00:28

 
  Build And Test
passed autotools #10824573

00:04:12

passed meson #10824574

00:02:53

passed meson-noglamor #10824575

00:01:36

passed mingw-cross-build #10824576

00:00:44