[ent vis] Improve model drawing.
[ent vis] Use scale vector instead of flip bools in poses
[ent vis] Sort model parts by z
[ent vis] Model::draw takes multiple skins.
[vis world] Skins only have one texture per app
[vis world] Give Skin::Apps a z_offset
[ent vis world] Try switching to positive depth = toward viewer
[ent vis] Finish fixing model rendering and depth testing and stuff.  Turns out glBlendEquation(GL_MAX) just skips the glBlendFunc.