xf86Crtc: keep transformation filters used by CRTCs around

Open Marco Trevisan requested to merge 3v1n0/xserver:keep-used-transformation-filters into master

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

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