backend/drm: add support for suggested output position prop

Virtualised outputs use the "suggested X" and "suggested Y" DRM props to
indicate their relative position on the host WM.

This change adds members `suggested_x` and `suggested_y` to `struct
wlr_output` which contain the value of these props. Also, a
`suggested_position` signal is added so that compositors can be notified
if there is a changed in the suggested position.

include/backend/drm/properties.h

@@ -17,6 +17,8 @@ union wlr_drm_connector_props {
 		uint32_t link_status; // not guaranteed to exist
 		uint32_t path;
 		uint32_t vrr_capable; // not guaranteed to exist
+		uint32_t suggested_x;
+		uint32_t suggested_y;
 
 		// atomic-modesetting only
 

include/wlr/interfaces/wlr_output.h

@@ -116,6 +116,13 @@ void wlr_output_update_custom_mode(struct wlr_output *output, int32_t width,
  * to notify compositors about the change.
  */
 void wlr_output_update_available_modes(struct wlr_output *output);
+/**
+ * Update the output suggested position.
+ *
+ * The backend must call this function when the suggested position is updated
+ * to notify compositors about the change.
+ */
+void wlr_output_update_suggested_position (struct wlr_output *output);
 /**
  * Update the current output status.
  *

include/wlr/types/wlr_output.h

@@ -128,6 +128,7 @@ struct wlr_output {
 	char model[16];
 	char serial[16];
 	int32_t phys_width, phys_height; // mm
+	int32_t suggested_x, suggested_y;
 
 	// Note: some backends may have zero modes
 	struct wl_list modes; // wlr_output_mode::link
@@ -171,6 +172,7 @@ struct wlr_output {
 		struct wl_signal enable;
 		struct wl_signal mode;
 		struct wl_signal available_modes;
+		struct wl_signal suggested_position;
 		struct wl_signal scale;
 		struct wl_signal transform;
 		struct wl_signal description;