work around uint16 limit for index buffers

WebGL restricts index buffers to uint16 type. That's not enough
to render large molecules and we quickly run out of indices. This
patch distributes the geometry across multiple vertex arrays. The
splitting has been properly implemented for trace, balls and sticks
and spheres render modes. Cartoon and tube will follow, but since
it's less of problem for these render modes, that can wait.

Fixes #26.