Add support for alpha-modifier-v1

(cherry picked from commit a0b3606)

Author: emersion

sway/server.c

@@ -9,6 +9,7 @@
 #include <wlr/config.h>
 #include <wlr/render/allocator.h>
 #include <wlr/render/wlr_renderer.h>
+#include <wlr/types/wlr_alpha_modifier_v1.h>
 #include <wlr/types/wlr_compositor.h>
 #include <wlr/types/wlr_content_type_v1.h>
 #include <wlr/types/wlr_cursor_shape_v1.h>
@@ -321,6 +322,7 @@ bool server_init(struct sway_server *server) {
 		&server->pointer_constraint);
 
 	wlr_presentation_create(server->wl_display, server->backend);
+	wlr_alpha_modifier_v1_create(server->wl_display);
 
 	server->output_manager_v1 =
 		wlr_output_manager_v1_create(server->wl_display);