Increase the "hit area" within menu items

[zachkinstner]

The standard menu item renderers provide a hit area along the outside edge of the item. This works well when you bring the cursor toward the menu arc from the side.

However, there are use cases where the cursor might move past this outer edge (toward the menu hand's palm). This can occur if the cursor moves too far horizontally, or is coming toward the menu arc from a "depth" direction. For the new "LeapLook" input module, you often look at the center of the menu item.

To improve this, add more selection points within each menu arc. This will improve the scenarios described above. Essentially, if the cursor is near to any part of a menu item, it should be able to generate a selection.

[zachkinstner]

This could probably use some optimization to reduce the number of "hit area" points that must be checked/measured. I made a TODO note in the code.