ddd574ef4f
DrmOutput's _file field was stored but unused; flip() reopened /scheme/drm/card0 on every page flip. Rename to drm_file, un- underscore, and use &self.drm_file in flip() to avoid the per-flip open+close round trip. This is the implementation of v6.0 plan Phase 3.5 'Fix page flip to keep DRM fd open'.