smurf: Fix spatial offset in FLT masking

The FLT masking was mis-placed in time by twice the padding plus any
apodisation. This could have seriously affected the working of FLT
masking if the the mask contains any small (less than twice the padding
plus apodisation) source regions.
smurf: FLT should re-order LUT model if any masking is needed

Not usually a problem, since the LUT model is usually in the correct
order anyway. But the new OFF model swaps the order, and so triggers a
FLT masking fault if used.
ast: Fix support for reading AIPS "GLS" projections.
ast: Document fix for GLS projections