Allow running kwin5 windowed in kwin6
In KWin 6 we dropped support for ZWP_LINUX_BUFFER_PARAMS_V1_FLAGS_Y_INVERT which meant that we didn't support the older KWin version. This makes development a bit harder when working with KWin 5 on KWin 6.
This change makes the "Y invert" happen in-process in a shadow framebuffer so we don't require the compositor to support it.
This change is intended for KWin 5, KWin 6 already handles this more gracefully.
Draft because I want to test it with our internal infrastructure