Poi3D models: correct flip axis on loading

[Gustl22]

I think vtm library works mirror-inverted at y-axis. I notice this with geometry buffer, too. (In code is z-axis, but as model is rotated around x-axis, it's actually the y-axis)
Flip gl front face cause of mirrored model.
Fixes #686.

[Gustl22]

Usually should switch over to libgdx y up convention, but it seems we use a z up coordinate system and fbx-conv always converts to the same output and the wiki seems obsolete. See libgdx/fbx-conv#102 like schedul-xor mentioned already.

[Gustl22]

Temporary close, cause work on a better implementation.

[Gustl22]

We have following coord systems (without warranty):

It seemed, we had these options for models:

• LibGdx Coord system (current implementation)
  • Export from .blend to .fbx with z-up (-y)-forward rhs, scale 0.01
  • Convert .fbx to .g3db with fbx-conv: automatically rotates 90 degree.
  • Vtm: invert rhs to lhs; (node rotation only seem to work, when gdx model is not already rotated)
• Vtm Coord system:
  • Export from .blend to .fbx with y-up z-forward rhs, scale 0.01
  • Convert .fbx to .g3db with fbx-conv: should not rotate from description
  • Vtm: invert rhs to lhs; no rotation

Second is not working, as fbx-conv somehow rotates anyway.

I tried a different approach to flip y axis of camera to avoid face flip in gl, no success. So existent PR should be the best solution right now.

I think somewhere I have a fallacy. I cannot explain why blender export z-up (-y)-forward rhs with experimental Apply transform results in upside down model.

However this config should work (also with Apply transform) and fbx-conv:

[devemux86]

Thanks, merged via bda4885.