Allow iterating over VoxelPalletes

[RiscadoA]

Problem

Currently iterating over palettes is very error prone considering that uint16_t are used as indices, and that its possible that a palette has a material on the last uint16_t value. Thus, VoxelPalette::size() returns the number of materials excluding the empty material (0). We end up having multiple places implementing a weird looking iteration.

Solution

Implement a VoxelPalette::Iterator and use it everywhere which iterates over it, in order to write the cursed logic only once.