VOXELFORMAT: VOX: missing palette color

[mgerhardy]

export the model and load back in magica voxel the pallete color first one is missing

The error is most likely in PaletteFormat::save

[AndrejCro]

Steps to reproduce:
Load pallete in vengi editor
Open vox file, select to map colors to the pallete
Trigger autosave
Open autosaved vox file in magica voxel
Pallete color at index 0 (1) is missing

[AndrejCro]

[mgerhardy]

The problem here is that the PaletteFormat::save function is not smart enough to search for unused palette color slots - and the provided palette includes 256 colors. Currently we try to find the closest match. This is fine, but we should also check for unused slots before. This functionality is there in SceneManager::nodeRemoveUnusedColors and should be moved out of the voxedit scope into a more global scope to be re-used in the voxelformat, too

[mgerhardy]

this improved the situation where a full 256 color palette should get mapped onto a palette that only supports 255 colors
because one of the colors is representing air (e.g. sandbox vxm or magicavoxel vox) - we are now not just looking for
a suitable replacement to get the good result, but we also try to first search for an unused slot in the palette that we
are then using for the slot that represents air in the target format/palette.

to work around this, make sure to use a properly formed palette file. E.g. for magicavoxel you should use the first slot
for a fully transparent pixel to indicate an empty slot - for sandbox vxm you should use the 256st color to be fully
transparent. If you assemble your palette that way you won't get any reordering or color reduction