[vulkan] Wait on device idle before deleting models

This fixes the textures (and presumably mesh data) being deleted while
still in use. Oddly, the wait was needed in both brush and alias models
(I expected brush to always come first).

libs/models/alias/vulkan_model_alias.c

@@ -83,6 +83,8 @@ vulkan_alias_clear (model_t *m, void *data)
 	aliashdr_t *hdr;
 	qfv_alias_mesh_t *mesh;
 
+	QFV_DeviceWaitIdle (device);
+
 	m->needload = true;	//FIXME is this right?
 	if (!(hdr = m->aliashdr)) {
 		hdr = Cache_Get (&m->cache);

libs/models/brush/vulkan_model_brush.c

@@ -71,6 +71,8 @@ static void vulkan_brush_clear (model_t *mod, void *data)
 	qfv_devfuncs_t *dfunc = device->funcs;
 	mod_brush_t *brush = &mod->brush;
 
+	QFV_DeviceWaitIdle (device);
+
 	for (int i = 0; i < brush->numtextures; i++) {
 		texture_t  *tx = brush->textures[i];
 		if (!tx) {