backend/drm: add drm_fb_copy()

Similar to drm_fb_move(), but leaves old as-is.

backend/drm/drm.c

@@ -467,10 +467,7 @@ static bool drm_crtc_commit(struct wlr_drm_connector *conn,
 	struct wlr_drm_crtc *crtc = conn->crtc;
 	bool ok = drm->iface->crtc_commit(conn, state, page_flip, flags, test_only);
 	if (ok && !test_only) {
-		drm_fb_clear(&crtc->primary->queued_fb);
+		drm_fb_copy(&crtc->primary->queued_fb, state->primary_fb);
-		if (state->primary_fb != NULL) {
-			crtc->primary->queued_fb = drm_fb_lock(state->primary_fb);
-		}
 		if (crtc->cursor != NULL && conn->cursor_pending_fb != NULL) {
 			drm_fb_move(&crtc->cursor->queued_fb, &conn->cursor_pending_fb);
 		}

backend/drm/fb.c

@@ -249,3 +249,10 @@ void drm_fb_move(struct wlr_drm_fb **new, struct wlr_drm_fb **old) {
 	*new = *old;
 	*old = NULL;
 }
+
+void drm_fb_copy(struct wlr_drm_fb **new, struct wlr_drm_fb *old) {
+	drm_fb_clear(new);
+	if (old != NULL) {
+		*new = drm_fb_lock(old);
+	}
+}

include/backend/drm/fb.h

@@ -17,6 +17,7 @@ bool drm_fb_import(struct wlr_drm_fb **fb, struct wlr_drm_backend *drm,
 void drm_fb_destroy(struct wlr_drm_fb *fb);
 
 void drm_fb_clear(struct wlr_drm_fb **fb);
+void drm_fb_copy(struct wlr_drm_fb **new, struct wlr_drm_fb *old);
 void drm_fb_move(struct wlr_drm_fb **new, struct wlr_drm_fb **old);
 struct wlr_drm_fb *drm_fb_lock(struct wlr_drm_fb *fb);