Did some polish and cleanup of the platform layer

Did final polish and cleanup of the platform layer since I want to actually finally cut a release relatively soon and I just did a lot of work there. Fixed things like useless tests, outdated or wrong comments and some platform specific quirks. Probably actually gonna cut a release soon (this time for real). Also added early rejections for BVH and CSM files, since they contain no model geometry (they can’t currently be displayed at all) and documented the change. The code is basically done (hopefully, I really don’t want to find another big gap that’s gonna take a week to close), I just want to correct anything wrong in the README, changelog, manpage or help menu and maybe make them look a little better before cutting a release and shipping.