render/drm_format_set: allocate using cap when duplicating

In wlr_drm_format_dup, allocate the new wlr_drm_format using cap instead
of len. This makes it so the cap field is up-to-date and the chunk of
memory isn't too small if we append new modifiers (we don't allow this
yet but might in the future).

render/drm_format_set.c

@@ -129,7 +129,7 @@ bool wlr_drm_format_set_add(struct wlr_drm_format_set *set, uint32_t format,
 
 struct wlr_drm_format *wlr_drm_format_dup(const struct wlr_drm_format *format) {
 	size_t format_size = sizeof(struct wlr_drm_format) +
-		format->len * sizeof(format->modifiers[0]);
+		format->cap * sizeof(format->modifiers[0]);
 	struct wlr_drm_format *duped_format = malloc(format_size);
 	if (duped_format == NULL) {
 		return NULL;