* fic gtk-doc typenames in cgif
* fix flatten clipping
flatten could produce out of range values if max_alpha was less than
the limit of the numeric range of the format
https://github.com/libvips/libvips/issues/2431
Fix two bugs:
- clip in casts from int32 and uint32 could overflow -- do these as gint64 now
- clip in casts from float to int could overflow since float32 can't
represent the full range of int32 without losing precision -- do these
as double
And add some more tests.
Thanks ewelot.
see https://github.com/libvips/libvips/issues/1922
