Some other software fixes/changes

Some other software fixes/changes I've made

* Change 1: single-sided linedef midtextures (read: not double-sided linedefs, but single-sided) now don't skew if Effect 2 is checked. Before, there wasn't any ability for software to disable skewing for them. =V
* Change 2: PolyObject back side midtextures now return to being where they should be. Had to make a hack to fix this though.

See merge request !86

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

Effect 2 (No Midtexture Skew) now toggles off skewing for midtextures on single-sided lines, which was what was intended for them to begin with apparently.

This means the current skewing-by-default effect isn't changed, and OpenGL's equivalent code doesn't have to be touched since apparently it was already like that.

"frontsector" in this part of the code isn't actually the polyobject's sector for back-side polyobject segs, it's the in-level sector the polyobject as a whole is being rendered in it turns out.

Red apparently left in code for single-sided linedefs to NOT skew their midtextures ...but it doesn't work because it doesn't stop the skewing code from running instead, regardless of whether Effect 1 is on or not. If it's decided single-sided line midtextures shouldn't do this though, the non-skew code could just as well be thrown out lol (or something else I guess?)

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