RHI dev: assert when resizing window

When running with RHI vulkan backend on Linux, and a window manager that does continuous updates during resizing, the attached sample app will quickly assert during resizing (dragging the right window edge left and right a few times).

Debugging shows it happens right after the "- bailing out due to failed beginFrame, wake Gui" line in qsgthreadedrenderloop (it tries to unlock a mutex that is not locked). beginFrame() has failed with the return code QRhi::FrameOpSwapChainOutOfDate.