present: Drop flip_idler member from present_vblank_rec

It's redundant with the pixmap member of struct xwl_present_event.

Acked-by: Olivier Fourdan <ofourdan@redhat.com>
5 jobs for present_wnmd-to-xwayland-present in 4 minutes and 51 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Docker Image
passed debian-buster #11669064

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  Build And Test
passed autotools #11669065

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passed meson #11669066

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passed meson-noglamor #11669067

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passed mingw-cross-build #11669068

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