add --straight flag to take model screenshots from head-on

[ox]

This diff adds a --straight flag (flexible on the naming), which can toggle the viewer.set_camera_from_bounds' straight argument. I have some models which are meant to be looked at straight-on, and this would directly solve that issue.

A more generalized solution would be to be able to specify camera rotation angles around the bounding box, but I'm not sure how that would jive with the count flag.

[bwasty]

Thanks! I changed the help string a bit because I consider the automatically positioned camera to be the default one (most sample files don't contain a camera).