output: avoid use of wlr_scene_output.WLR_PRIVATE.index

We were only using it to allow quick bitset comparisons of sets of
outputs (such as view->outputs). We can maintain our own bit IDs for
this purpose and avoid using the private wlroots field.

Note: from my reading of wlr_scene_output_create(), it appears to
always take the lowest unused index, resulting in aggressive re-use of
index values when outputs are disconnected and reconnected. I've tried
to make re-use as infrequent as possible. This could theoretically
reduce the chance of a mix-up in view_update_outputs(), although I'm
not aware of any practical scenario where it matters.

v2: prevent adding more than 64 outputs

include/labwc.h

@@ -257,6 +257,7 @@ struct server {
 	struct wl_list outputs;
 	struct wl_listener new_output;
 	struct wlr_output_layout *output_layout;
+	uint64_t next_output_id_bit;
 
 	struct wl_listener output_layout_change;
 	struct wlr_output_manager_v1 *output_manager;

include/output.h

@@ -33,6 +33,18 @@ struct output {
 	struct wl_listener frame;
 	struct wl_listener request_state;
 
+	/*
+	 * Unique power-of-two ID used in bitsets such as view->outputs.
+	 * (This assumes there are never more than 64 outputs connected
+	 * at once; wlr_scene_output has a similar limitation.)
+	 *
+	 * There's currently no attempt to maintain the same ID if the
+	 * same physical output is disconnected and reconnected.
+	 * However, IDs do get reused eventually if enough outputs are
+	 * disconnected and connected again.
+	 */
+	uint64_t id_bit;
+
 	bool gamma_lut_changed;
 };
 

include/view.h

@@ -158,7 +158,7 @@ struct view {
 	 * This is used to notify the foreign toplevel
 	 * implementation and to update the SSD invisible
 	 * resize area.
-	 * It is a bitset of output->scene_output->index.
+	 * It is a bitset of output->id_bit.
 	 */
 	uint64_t outputs;