x11 backend: Do not set a back pixel on windows

Before this commit, the x11 server would fill any exposed area with
white before the wlroots x11 backend got a chance to do anything. This
was e.g. visible when running rootston and resizing the window: When the
window becomes larger, the new area is filled with black.

By just not setting a back pixel value, this commit gets rid of this
behaviour.

Signed-off-by: Uli Schlachter <psychon@znc.in>

backend/x11/output.c

@@ -114,9 +114,8 @@ struct wlr_output *wlr_x11_output_create(struct wlr_backend *backend) {
 		wl_list_length(&x11->outputs) + 1);
 	parse_xcb_setup(wlr_output, x11->xcb_conn);
 
-	uint32_t mask = XCB_CW_BACK_PIXEL | XCB_CW_EVENT_MASK;
-	uint32_t values[2] = {
-		x11->screen->white_pixel,
+	uint32_t mask = XCB_CW_EVENT_MASK;
+	uint32_t values[] = {
 		XCB_EVENT_MASK_EXPOSURE |
 		XCB_EVENT_MASK_KEY_PRESS | XCB_EVENT_MASK_KEY_RELEASE |
 		XCB_EVENT_MASK_BUTTON_PRESS | XCB_EVENT_MASK_BUTTON_RELEASE |