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

Basically this makes sure numwadfiles is updated before loading the SOC/Lua scripts, so if a Lua script calls COM_BufInsertText with the contents "addfile scr_mysticrealm.wad" it can't overwrite the last written wadfile slot! Not that COM_BufInsertText really should be used like that to begin with

Reviewed by @RedEnchilada

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

This fixes certain sector specials and linedef executor specials etc not accounting for players/mobjs touching sloped FOFs

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

(assuming it wasn't an intentional behaviour thing of course)