output: add wlr_output_state.allow_artifacts

When starting up, the compositor might call wlr_output_set_mode()
with a mode which is already the current one. wlroots will detect
this and make the wlr_output_set_mode() call a no-op. During the
next wlr_output_commit() call, wlroots will perform an atomic
commit without the ALLOW_MODESET flag.

This is an issue, because some drivers need ALLOW_MODESET even if
the mode is the same. For instance, if the FB stride or modifier
changed, some drivers require a modeset.

Add a new flag "allow_artifacts" which is set when the compositor
calls mode-setting functions. Use this flag to figure out whether
we want to perform atomic commits with ALLOW_MODESET.

(The name "allow_artifacts" is picked because ALLOW_MODESET is a
misnomer, see [1].)

[1]: https://patchwork.freedesktop.org/patch/505107/

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3499

backend/drm/drm.c

@@ -384,8 +384,7 @@ static void drm_connector_state_init(struct wlr_drm_connector_state *state,
 		const struct wlr_output_state *base) {
 	memset(state, 0, sizeof(*state));
 	state->base = base;
-	state->modeset = base->committed &
+	state->modeset = base->allow_artifacts;
-		(WLR_OUTPUT_STATE_ENABLED | WLR_OUTPUT_STATE_MODE);
 	state->active = (base->committed & WLR_OUTPUT_STATE_ENABLED) ?
 		base->enabled : conn->output.enabled;
 

include/wlr/types/wlr_output.h

@@ -79,6 +79,9 @@ enum wlr_output_state_mode_type {
  */
 struct wlr_output_state {
 	uint32_t committed; // enum wlr_output_state_field
+	// Set to true to allow temporary visual artifacts (e.g. black screen) while
+	// the update is being applied
+	bool allow_artifacts;
 	pixman_region32_t damage; // output-buffer-local coordinates
 	bool enabled;
 	float scale;

types/output/state.c

@@ -4,6 +4,7 @@ void wlr_output_state_set_enabled(struct wlr_output_state *state,
 		bool enabled) {
 	state->committed |= WLR_OUTPUT_STATE_ENABLED;
 	state->enabled = enabled;
+	state->allow_artifacts = true;
 }
 
 void wlr_output_state_set_mode(struct wlr_output_state *state,
@@ -11,6 +12,7 @@ void wlr_output_state_set_mode(struct wlr_output_state *state,
 	state->committed |= WLR_OUTPUT_STATE_MODE;
 	state->mode_type = WLR_OUTPUT_STATE_MODE_FIXED;
 	state->mode = mode;
+	state->allow_artifacts = true;
 }
 
 void wlr_output_state_set_custom_mode(struct wlr_output_state *state,
@@ -20,6 +22,7 @@ void wlr_output_state_set_custom_mode(struct wlr_output_state *state,
 	state->custom_mode.width = width;
 	state->custom_mode.height = height;
 	state->custom_mode.refresh = refresh;
+	state->allow_artifacts = true;
 }
 
 void wlr_output_state_set_scale(struct wlr_output_state *state, float scale) {