ogt_vox: groups and instances are not correctly ordered

[dougbinks]

MagicaVoxel uses instance order to control render priority, such that a wrongly ordered scene may not render correctly.

However ogt_vox.h currently writes all groups first then all instances, which breaks the ordering.

I have a branch with a fix which I will submit, but since this may need some discussion so I'm opening an issue first to ensure the problem is tracked separately from the proposed solution.

[dougbinks]

This is affected by #40