The tree is truncated such that it does not contain boolean value-nodes
all the way to the leafs. Instead, leafs store bitmasks of a specified
size.

Also contains algorithms to efficiently compute the number and bounding box
of set pixels in such bitmasks.

A RandomAccessible<NativeBoolType> that grows on demand.
SparseBitmask.tree() provides the underlying SparseBitmaskNTree.
SparseBitmask.region() provides a view as IterableRegion, with efficient
computation of bounding box and iteration of set pixels.

The functionality of Bitmask is very much independent of the class Tree.
It should be an independent class.

Only on bit of SparseBitmask is true.
But SparseBitmask.region().cursor() seems to have two elements?

* numSet -> numberOfOnes (numSet is too ambiguous. Is it a set of numbers?)
* leafDims -> dims
* bytes -> data (The javadoc comment suggests this renaming.)

Add method byteMask similar to byteIndex to reduce code duplication.

The design of this class is specific to it's use in Tree this
should be reflected by the name

The sparse bitmaps in Labkit use list's of coordinates to store only the
white pixels.

If a set operation sets NodeData.children or NodeData.bitmask to zero,
get will return NodeData.value.

That way GrowableTree and Tree can be better distinguished.
Also reading Tree becomes thread-safe. The tree can always be red in any
thread, only the write operations need to be synchronized.

This has a huge performance improvement for cuncurrent reading.