backend-drm: prepare virtual output API for heterogeneous outputs

Stop plugins from overwriting the struct weston_output::destroy vfunc,
as that will be used by backends to recognize their outputs.
Instead, pass a plugin-specific destroy callback when creating the
virtual output.

See: https://gitlab.freedesktop.org/wayland/weston/-/issues/268

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

libweston/backend-drm/drm-internal.h

@@ -579,6 +579,7 @@ struct drm_output {
 	struct wl_event_source *pageflip_timer;
 
 	bool virtual;
+	void (*virtual_destroy)(struct weston_output *base);
 
 	submit_frame_cb virtual_submit_frame;
 };

libweston/backend-drm/drm-virtual.c

@@ -259,6 +259,9 @@ drm_virtual_output_destroy(struct weston_output *base)
 
 	drm_output_state_free(output->state_cur);
 
+	if (output->virtual_destroy)
+		output->virtual_destroy(base);
+
 	free(output);
 }
 
@@ -324,7 +327,8 @@ drm_virtual_output_disable(struct weston_output *base)
 }
 
 static struct weston_output *
-drm_virtual_output_create(struct weston_compositor *c, char *name)
+drm_virtual_output_create(struct weston_compositor *c, char *name,
+			  void (*destroy_func)(struct weston_output *))
 {
 	struct drm_output *output;
 	struct drm_backend *b = to_drm_backend(c);
@@ -343,6 +347,7 @@ drm_virtual_output_create(struct weston_compositor *c, char *name)
 	}
 
 	output->virtual = true;
+	output->virtual_destroy = destroy_func;
 	output->gbm_bo_flags = GBM_BO_USE_LINEAR | GBM_BO_USE_RENDERING;
 
 	weston_output_init(&output->base, c, name);