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