Horoball viewer glitch

[unhyperbolic]

If the current peripheral framing is far from the one that minimizes the length of the basis curves, then the cusp neighborhood view can be missing various horoballs and cells of the triangulation and Ford domain. Here is an example, created by:

M = snappy.Manifold('m004')
M.set_peripheral_curves([[1, 0], [18, 1]])
N = M.cusp_neighborhood().view() 

Another example is K14n18339; from the default position, just move to the right to see this image:

The root cause is likely in CyOpenGL.pyx, possibly in the build_shifts method of HoroballScene.

Reported by Saul Schleimer (https://bitbucket.org/t3m/snappy/issues/18/horoball-viewer-glitch).