Rework user data handling

[davids91]

• Most of the voxels have empty user data, which wastes a significant amount of space.
• Storing arbitrary, or relatively large user data better enables creative freedom
• color_palette doesn't store user data, so in case of Solid bricks, user data is lost on the GPU implementation (this is tech debt)
• It needs to be reconsidered if all voxels need to have user data, or if it's adequate to store user data in bricks only

To account for this, the storage of user data should be re-considered and it needs to be stored separately from voxel data.