Travis ci

Ok, now SRB2 on travis-ci is tested to be compiled against 17 buildsets

Now, what am I doing wrong?

See merge request !74

Significant rework of main seg-rendering code, to eliminate the possibility of drawing off-screen and crashing the game as result

NOTE: HOMs sometimes appear in the sky in maps like AGZ (map40), so this isn't completely fine yet. I'll fix that later