viewporter: remove crop and scale state upon destruction

According to the viewport protocol, upon wp_viewport::destroy():

> The associated wl_surface's crop and scale state is removed.
> The change is applied on the next wl_surface.commit.

Therefore, wp_viewport_destroy(viewport) should remove all viewport state.

Currently, wlroots does not remove the crop and scale state. Instead, a
client must do:

    wl_fixed_t clear = wl_fixed_from_int(-1);
    wp_viewport_set_source(viewport, clear, clear, clear, clear);
    wp_viewport_set_destination(viewport, -1, -1);
    wp_viewport_destroy(viewport);

This commit adds the necessary logic into viewport_destroy and makes
wlroots comply with the protocol.

types/wlr_viewporter.c

@@ -95,6 +95,12 @@ static void viewport_destroy(struct wlr_viewport *viewport) {
 	if (viewport == NULL) {
 		return;
 	}
+
+	struct wlr_surface_state *pending = &viewport->surface->pending;
+	pending->viewport.has_src = false;
+	pending->viewport.has_dst = false;
+	pending->committed |= WLR_SURFACE_STATE_VIEWPORT;
+
 	wl_resource_set_user_data(viewport->resource, NULL);
 	wl_list_remove(&viewport->surface_destroy.link);
 	wl_list_remove(&viewport->surface_commit.link);