For interlaced vertically subsampled images we need to combine alternating
chroma lines with alternating luma lines. That is line 0 and 2 are combined
with the first line of chroma samples and line 1 and 3 with the second line
of chroma samples.
See also: https://bugzilla.gnome.org/show_bug.cgi?id=588535
Add an unpack option to specify what to do with the least significant bits of
the destination when the source format has less bits than the destination. By
default we will now copy the most significant bits of the source into the least
significant bits of the destination so that the full color range is represented.
Add an option to leave the extra destination bits 0, which may be faster and
could be compensated for in the element algorithm.
Use a separate variable to describe the amount of lines that will be used in
packing instead of abusing the h_sub variable. Some formats might have no
subsampling but need to operate on multipe lines.
