Unaccelerated rendering with GPUP shows stale content.

https://bugs.webkit.org/show_bug.cgi?id=255112
<rdar://107717934>

Reviewed by Simon Fraser.

Only IOSurface/MachSendRight types can be cached using CAIOSurface, otherwise
we end up caching a stale CGImage copied from a ShareableBitmapHandle.

* Source/WebKit/Shared/RemoteLayerTree/RemoteLayerBackingStore.mm:
(WebKit::RemoteLayerBackingStoreProperties::updateCachedBuffers):

Canonical link: https://commits.webkit.org/262686@main

Source/WebKit/Shared/RemoteLayerTree/RemoteLayerBackingStore.mm

@@ -693,7 +693,7 @@ static bool hasValue(const ImageBufferBackendHandle& backendHandle)
 {
     Vector<RemoteLayerTreeNode::CachedContentsBuffer> cachedBuffers = node.takeCachedContentsBuffers();
 
-    if (contentsType != LayerContentsType::CachedIOSurface || !m_frontBufferIdentifier)
+    if (contentsType != LayerContentsType::CachedIOSurface || !m_frontBufferIdentifier || !m_bufferHandle || !std::holds_alternative<MachSendRight>(*m_bufferHandle))
         return;
 
     cachedBuffers.removeAllMatching([&](const RemoteLayerTreeNode::CachedContentsBuffer& current) {