output: turn make/model/serial into char *

This allows the make/model/serial to be NULL when unset, and allows
them to be longer than the hardcoded array length.

This is a breaking change: compositors need to handle the new NULL
case, and we stop setting make/model to useless "headless" or
"wayland" strings.

include/wlr/types/wlr_output.h

@@ -122,9 +122,7 @@ struct wlr_output {
 
 	char *name;
 	char *description; // may be NULL
-	char make[56];
-	char model[16];
-	char serial[16];
+	char *make, *model, *serial; // may be NULL
 	int32_t phys_width, phys_height; // mm
 
 	// Note: some backends may have zero modes