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.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>