Fixes https://github.com/libgdx/libgdx/issues/2148
Since https://github.com/libgdx/fbx-conv/commit/de8d246671ca42a5a8fe1232002de2c12583619a triangulating geometry (and needed for fbx sdk 2015: also replacing the previous geometry) causes the array to be reordered. Therefore need to split in two iterations, first triangulate and then prefetch.
Also added a warning, so it isn't possible that a geometry is skipped without logging a warning.