note X-Trans CFA alignment is different than Bayer

It may well be better for X-Trans to conform to the Bayer example, so
note this difference. This is especially true as, once the image
rotation happens in the flip IOP, it will no longer be necessary to
determine the shift of the X-Trans CFA depending on image rotation.
eliminate X-Trans linear demosaic from UI

Linear is fast, but not that good quality. The actual
xtrans_lin_interpolate() code needs to remain, as
xtrans_vng_interpolate() uses it.

Conflicts:
	doc/usermanual/darkroom/modules/basic/demosaic.xml
	src/iop/demosaic.c