xf86Crtc: keep transformation filters used by CRTCs around

Make a temporary copy of the filter and restore it once we might need
them, as they might get free'd by the server in the mean time.

We keep the copy flag inside the transformation itself so that we don't
need to do any ABI change.

Fixes #14
2 jobs for keep-used-transformation-filters in 1 minute and 23 seconds (queued for 98 minutes and 49 seconds)