redbear-compositor: keep DRM fd open across page flips (Gap 3.5)

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'.

local/recipes/wayland/redbear-compositor/source/src/display_backend.rs

@@ -201,7 +201,7 @@ mod drm_backend {
         pub buffers: Vec<(usize, usize)>,
         fb_ids: Vec<u32>,
         pub current: AtomicUsize,
-        _file: File,
+        drm_file: File,
     }
 
     impl DrmOutput {
@@ -420,7 +420,7 @@ mod drm_backend {
                 buffers,
                 fb_ids,
                 current: AtomicUsize::new(0),
-                _file: file,
+                drm_file: file,
             })
         }
 
@@ -434,9 +434,7 @@ mod drm_backend {
             let mut buf = Vec::with_capacity(12);
             buf.extend_from_slice(&(DRM_IOCTL_MODE_PAGE_FLIP as u64).to_le_bytes());
             buf.extend_from_slice(&fb_id.to_le_bytes());
-            if let Ok(mut f) = File::open("/scheme/drm/card0") {
-                let _ = f.write_all(&buf);
-            }
+            let _ = (&self.drm_file).write_all(&buf);
             self.current.store(next, Ordering::Relaxed);
         }