Software crashes fix

This branch SHOULD fix the many crashes people have reported lately that all point to the software renderer. Simply put, the software renderer allowed stuff to be drawn out of the screen even though that wasn't safe, and even the existing checks to prevent that didn't work.

If you saw me worrying about the sky HOMs I discovered in AGZ earlier in the commits for this branch, don't worry - it turns out that issue already existed in 2.1.15's srb2win.exe (and probably srb2dd.exe too) anyway, the changes in this branch didn't cause them. Hopefully nothing else broke then.

See merge request !75

Fix for rendering sprites in skyboxes

This branch fixes the visual glitches one sees with sprites for objects within skyboxes, typically with parts of or all of some sprites disappearing from view even though they shouldn't be.

See merge request !84

OpenGL slope fixes again

This branch just adds the relevant code for OpenGL to properly check slopes regarding "closed door" segs (those between two sectors that cannot be crossed normally) and "window" segs (those between two sectors of differing plane heights that you CAN cross, which would probably display top or bottom textures), so you don't get HOMs when the the slopes create a closed door even though the normal sector heights wouldn't or something.

(if you couldn't understand that, my slopes test map shows what I mean to the right of the player start: https://dl.dropboxusercontent.com/u/25409000/2.1/mi-slopetest.wad)

See merge request !81

OpenGL: Fix lower unpegged texture offset, fix lower unpegged + effect 1 so the texture actually skews

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