Shader access for segment property values

Currently neuroglancer shaders do not have access to segment properties for skeletons. This prevents us from doing useful things like modulating the appearance of objects based on segment-level properties.

For example: adjusting the skeleton shader to show neurons in different colors based on a segment property, or changing the way an axons or dendrites are visualized based on a segment property.

Mesh shaders also do not have segment property access, so one cannot modulate the appearance of a mesh based on segment properties.

Separately, segment properties are not possible to set via a local json state, and so dynamic modulation of a visualization of individual segments is not possible beyond 'visible/not-visible'.

cc @fcollman, @pedson