ogt_vox : materials are not merged correctly

[jibjoub]

Currently, the materials are well loaded with load_vox_scene but are not merged correctly in ogt_vox_merge_scenes. Thus no materials other than diffuse (default) exist in the merged scene.

[jpaver]

By the way, this PR has been merged: #39 and is a great start.

Currently it finds the palette entry that has the closest color to the one expected and then copies over the material properties for that, but it doesn't really try to match materials at all. This would be problematic If we have palette entries with identical colors but different materials.

To support this properly, we'd have to modify find_exact_color_in_palette and find_closest_color_in_palette to support closest material matching in addition to color matching.