Add new splat mode, add hiding support

[slimbuck]

Fixes: #50, #29, #47

This PR:

• introduces a new rendering/selection mode called "Rings", which allows selection by splat extents instead of splat centers
• adds an option to hide selected splats and unhide them
• adds a new picker selection tool to extend rect and brush
• uses a state array to track each splat state, instead of updating splat opacity
• double click the scene to center the orbit camera around the target point
• update npm dependencies, including engine v1.70.0