1-level quad tree optimization.
Viewport partitioning optimization.

To reduce inspecting DisplayNodes which do not have to be considered.
Adds a comment.
Layer will have viewport rect and world size.
Fixes a bug in removing Node from SubLayer.
Merge branch 'feature/streaming' into optimize/viewport_partitioning

Conflicts:
	src/Eye/Layer.jsx
Fixes incomplete merge.
Always render 9 SubLayers surrounding.
Fixes Layer.updateSubLayer bugs.
Clean up setting world size to Layer constructor.
Preserves backward compatibility.
Merge pull request #53 from mootoh/optimize/viewport_partitioning

Viewport Partitioning Optimization