backend/drm: Add async page flip support to legacy

Atomic doesn't support such flags yet.

include/backend/drm/drm.h

@@ -114,6 +114,8 @@ struct wlr_drm_backend {
 	uint64_t cursor_width, cursor_height;
 
 	struct wlr_drm_format_set mgpu_formats;
+
+	bool supports_tearing_page_flips;
 };
 
 struct wlr_drm_mode {

include/wlr/types/wlr_output.h

@@ -89,6 +89,12 @@ struct wlr_output_state {
 	enum wl_output_subpixel subpixel;
 
 	struct wlr_buffer *buffer;
+	/* Request a tearing page-flip. When enabled, this may cause the output to
+	 * display a part of the previous buffer and a part of the current buffer at
+	 * the same time. The backend may reject the commit if a tearing page-flip
+	 * cannot be performed, in which case the caller should fall back to a
+	 * regular page-flip at the next wlr_output.frame event. */
+	bool tearing_page_flip;
 
 	enum wlr_output_state_mode_type mode_type;
 	struct wlr_output_mode *mode;