Multiple drawings in one file

[my2iu]

Make a utility for putting multiple models in a single glTF file so that they can be transferred more easily. See if there are any performance wins possible by having a single buffer hold the data for multiple meshes simultaneously, similar to a spritesheet, but for 3d models.

Also make a separate utility for putting opaque parts of a mesh together and possibly flatten things more and possibly rearranging things to be front-to-back (as opposed to forcing users to go into Omber and reexport things)