Cannot output individual images of more than one structure view attached to one open alignment

When attempting to output structure images of multiple structures opened for one or more sequences in a single alignment, each structure has its image written over all of the output structure image filenames.

e.g.
jalview --open alignment.fa \
--structure struc1.pdb --seqid 101 --structureimage struc1.png \
--srtucture struc2.pdb --seqid 101 --structureimage struc2.png \
--all --overwrite

will result in the image of struc1.pdb first being written to BOTH struc1.png and struc2.png, and THEN
the image of struc2.pdb will be written to BOTH struc1.png and struc2.png.

--structureimage should be limited after a --structure argument up to the next --structureimage, or next --open.

This can probably be done relatively easily by adding a Type.STRUCTURE (like the Type.IMAGE, although --structureimage will be both, with Opt.PRIMARY applying to both -- might be worth adding an Opt.STRUCTUREPRIMARY, Opt.IMAGEPRIMARY though this becomes slightly less generic).