fix slight glitch in last child delete from larger voxel

This fixes the issue described here: fixes #1902

Here's what was happening:
* first of all, when the first child was deleted we call splitChildren() which creates smaller children with the same color as the parent. We should at this point set the color for the parent to "no color"
* additionally when we set the color of a node, if we set it to a non-color (last element = 0) we should not set it's density to 1
* finally, in the interface, we do a "blink effect" on voxels that we click on... but this effect is implemented by changing the color of the voxel in the local tree... this effect was bleeding through to the clicked on voxel to cause the parent's color to be set, even though we're in the process of deleting it.