LeaveVT/EnterVT cycles will free/realloc shadow frame buffers. Because of
this, the presense/absence of that data is insufficient to know whether
the screen function wrappers are necessary. Instead, the 'transform_in_use'
flag should be used.
This patch also adds 'xf86RotateFreeShadow' for drivers to use at LeaveVT
time to free the rotation data; it will be reallocated on EnterVT.