SDL2 i_video.c code cleanup

Made some code look a bit neater, removed the need for some SDL2STUBs Fury placed, vid_info console command actually kind of has a use again, etc

The last time this was experimented with, my mouse became hideously less sensitive in SRB2 and the change ended up never getting merged. I'm totally gonna be cautious about this change and if it feels wrong again I'll the entire branch for it.

If you're referring to https://github.com/STJr/SRB2/pull/84 on GitHub yeah, this should not affect mouse sensitivity but instead replaces a hack to make the mouse movement purely "relative" with SDL's own function for doing that properly. In other words it's taking a totally different approach from what Red was doing there (Red was tweaking the x/y mouse movement values independently of SDL/Directdraw/etc, I'm directly tweaking the SDL code to handle mouse movement better in the first place).

Ultimately the result of what I'm doing should be mostly the same as before, except a few minor quirks with mouse behaviour (such as how moving the mouse back into the window can suddenly cause the camera to instantly move all the way up/down or something, and hopefully the original issue Red was trying to fix that was reported here: http://mb.srb2.org/showthread.php?t=41461) should be fixed.

Currently I do find sometimes SRB2 takes precedence over other windows when you don't expect it to, like when clicking on something else in the taskbar... wonder if "grabbing" the mouse actually fixes the side effects of that or not, that's something for me to test later I suppose?

Status changed to merged

mentioned in commit 64801a1a