Document vtex

[JJL772]

VTex doesn't have a lot of documentation and it leads to some confusion about how it works. Paint points include:

1. Cubemap creation. Needs 4 wide 3 high grid of images, aligned in a T-shape like this:

Image	Position (x,y)
Right	3,2
Left	1,1
Back	2,1
Front	0,1
Up	3,0
Down	3,2

2. Supported keys in vtex configuration files: skybox, skyboxcropped, cubemap, startframe, endframe, volumetexture, bumpscale, pointsample, trilinear, clamps, clampu, border, anisotropic, dxt5, nocompress, normal, normalga, invertgreen, ssbump, nomip, allmips, mostmips, nonice, nolod, procedural, alphatest, alphatest_threshhold, alphatest_hifreq_threshhold, rendertarget, numchannels, singlecopy, oneovermiplevelinalpha, premultalphabymiplevelfraction, premultalphabymiplevelfraction_maxalphamiplevel, premultalphabymiplevelfraction_minalphaperpixel, premultcolorbyoneovermiplevel, normaltodudv, compute2dgradient, stripalphachannel, stripcolorchannel, normalalphatodudvluminance, dudv, reduce, reducex, reducey, maxwidth, maxheight, alphatodistance, distancespread, pfmscale, pfm, psd, displacementwrinkle, specvar, manualmip, mipblend, srgb, secondary_target_pathsubs, secondary_target_reduce
3. General vtex usage needs to be documented too, not many people seem to use the tool because VTFEdit exists.