ogt_vox: split fatal errors and warnings

[mgerhardy]

some ogt_asserts are no hard errors but only a warning. This PR should allow the user code to provide a ogt_warn functionality to prevent triggering an assert in such a scenario.

By default ogt_warn is set to ogt_assert.

Fixes issue #52

[mgerhardy]

I've left it as draft for now - because the locations of ogt_warn are open for discussion.