vk: avoid early destroy of renderbuffers' backing textures

src/rhi/qrhivulkan.cpp

@@ -3851,6 +3851,7 @@ void QVkRenderBuffer::release()
 
     if (backingTexture) {
         Q_ASSERT(backingTexture->lastActiveFrameSlot == -1);
+        backingTexture->lastActiveFrameSlot = e.lastActiveFrameSlot;
         backingTexture->release();
     }