this isn't a dosbox question and no. You can't improve Shadow Caster by using In Pursuit of Greed code, and finally the usage terms of the In Pursuit of Greed code is non-commercial educational use only, so legally you can't glue in GPL stuff from the more advanced id Tech derivative sources out there.

Hacking in VESA SVGA support is no trivial thing and this better not lead to a bug-every-dos-programmer-on-the-internet quest like your unsuccessful 'can u upload incontinence of blackmarsh' demands in EVERY. SINGLE. POWERVR. VIDEO.