Fix segfault in wlr_output_manager_v1_set_configuration

Calling wlr_output_manager_v1_set_configuration with an enabled output
and a NULL mode is incorrect if the output doesn't support modes.

When DPMS'ing an output, wlr_output_enable(output, false) is called.
This de-allocates the CRTC and sets wlr_output.current_mode to NULL.

Because we mark DPMS'ed outputs as enabled, we also need to provide a
correct output mode. Add a field to sway_output to hold the current
mode.

Closes: https://github.com/swaywm/wlroots/issues/1867

include/sway/output.h

@@ -32,6 +32,8 @@ struct sway_output {
 	int lx, ly; // layout coords
 	int width, height; // transformed buffer size
 	enum wl_output_subpixel detected_subpixel;
+	// last applied mode when the output is DPMS'ed
+	struct wlr_output_mode *current_mode;
 
 	bool enabled, configured;
 	list_t *workspaces;

sway/config/output.c

@@ -278,6 +278,7 @@ bool apply_output_config(struct output_config *oc, struct sway_output *output) {
 		sway_log(SWAY_ERROR, "Failed to modeset output %s", wlr_output->name);
 		return false;
 	}
+	output->current_mode = wlr_output->current_mode;
 
 	if (oc && oc->scale > 0) {
 		sway_log(SWAY_DEBUG, "Set %s scale to %f", oc->name, oc->scale);

sway/desktop/output.c

@@ -662,6 +662,7 @@ static void update_output_manager_config(struct sway_server *server) {
 			root->output_layout, output->wlr_output);
 		// We mark the output enabled even if it is switched off by DPMS
 		config_head->state.enabled = output->enabled;
+		config_head->state.mode = output->current_mode;
 		if (output_box) {
 			config_head->state.x = output_box->x;
 			config_head->state.y = output_box->y;

sway/tree/output.c

@@ -269,6 +269,7 @@ void output_disable(struct sway_output *output) {
 
 	output->enabled = false;
 	output->configured = false;
+	output->current_mode = NULL;
 
 	arrange_root();
 }