I booted up SRB2 R7208 For you, and tried out software mode fullscreened 1366x768.
The result was the same, I still get strange diagonal lines like before. Snd when I tried to use Printscreen to get a screencap, the printscreen return was nothing like the actual view. In fact, it was very darkened.
...
But I can still share you what it looked like:
Printscreen tries to tell that something just isn't right with the game.
But.. when using the screenshot command.. It appears to be just fine.
So, it's really hard for me to see this error thanks to, well.. overlapping errors!
EDIT:
The thing is that the interface code should had filter out any video mode that more then 1920 in width or 1200 in height, so you should had been about to go into 2048x1536 mode, the software renderer does not work that high up (we need to alloc alot more base memory so the software can work with higher resolutions).
I never tried going into software with the really big mode. I could try and force it in the Net Launcher if you want.
EDIT AGAIN:
I tried SRB2 v1.09.4 in the odd video mode, and it worked fine, it just sliced off 6 pixels to let it work.