Add "v.dupx_splitscreen()" for Lua HUD stuff

This merge requests depends on !620 (closed) to be merged first.

However, while that merge request is probably suitable for the "master" branch as it doesn't change gameplay itself in any way, this merge request changes some Lua behaviour, and should therefore probably go to the "next" branch, not "master".

This merge request adds "v.dupx_splitscreen()" and "v.dupy_splitscreen()" for HUD stuff, which have a similar purpose to the already-existing "v.dupx()" and "v.dupy()", except for getting the scale of V_PERPLAYER elements while in splitscreen (which can be set using !620 (closed)'s "gui_maxscale_splitscreen"), rather than the otherwise-global element size (which can be set using !620 (closed)'s "gui_maxscale").

This merge request doesn't add "v.squish_splitscreen()" or anything else to check !620 (closed)'s "gui_squish_splitscreen". I can add that in if that's wanted, but I won't do so unless I'm asked to.