Sample reordering of annotation file

[pd3]

As mentioned in #1784 (comment), currently it is not possible to override the order of VCF sample names with -s/-S. Ideally these two commands would give the following outputs:

$ bcftools annotate file.vcf -a annots.tab.gz  -c CHROM,POS,FMT/TEST -s smpl1,smpl2 | bcftools query -f'[ %TEST]\n'
1 2

$ bcftools annotate file.vcf -a annots.tab.gz  -c CHROM,POS,FMT/TEST -s smpl2,smpl1 | bcftools query -f'[ %TEST]\n'
2 1