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/lib.rs

@@ -112,10 +112,20 @@ impl ItemGraphicsCacheEntry {
 struct ItemGraphicsCache(RenderingCache<Option<ItemGraphicsCacheEntry>>);
 
 impl ItemGraphicsCache {
+    /// Convenience method for releasing an item's cached graphics data.
     fn release(&mut self, item_data: &CachedRenderingData) {
         item_data.release(&mut self.0);
     }
 
+    /// Clears the entire graphics cache. This is needed when for example loosing
+    /// the GL context (when unmapping a window) and destroying the canvas.
+    fn clear(&mut self) {
+        self.0.clear();
+    }
+
+    /// Convenience method that will return what's in the item's graphics cache
+    /// and call update_fn if the cache is outdated and needs refreshing. If
+    /// update_fn is called, the data is persisted in the cache.
     fn ensure_up_to_date(
         &mut self,
         item_data: &CachedRenderingData,