Fix disappearing images or panics when hiding and showing a window with wayland

Properly release GL resource when unmapping a window and detect stale
cache indices using a generation count. This fixes the disappearing
images (GL textures would be invalid and need to be re-created) as well
as the panics when an item's cache index ended up being re-used because
some other item was drawn first and allocated that indices. The latter
is fixed by using a generational counter on the cache that's bumped when
clearing, instead of a single "cache_ok" bool.

sixtyfps_runtime/rendering_backends/gl/graphics_window.rs

@@ -209,6 +209,8 @@ impl GraphicsWindow {
     /// Removes the window from the screen. The window is not destroyed though, it can be show (mapped) again later
     /// by calling [`PlatformWindow::map_window`].
     fn unmap_window(self: Rc<Self>) {
+        // Release GL textures and other GPU bound resources.
+        self.graphics_cache.borrow_mut().clear();
         self.map_state.replace(GraphicsWindowBackendState::Unmapped);
         /* FIXME:
         if let Some(existing_blinker) = self.cursor_blinker.borrow().upgrade() {