Process partial segments

[thorstenhater]

arb::pwlin can return an arb::msegment{id, p', q'} describing a subsection of the 'original' segment (id, p, q) such that
p' = q + u(q - p) and q' = p + v(q - p) for u, v: [0, 1].

At the moment, the renderer just inspects the id and returns the full segment regardless of any points.

Idea

1. Add a field f to all points in the renderer that is 0 on the proximal and 1 on the distal end. Ensure linear interpolation.
2. Pass a pair (u, v) to each frustrum when rendering based on the fractions above. Ensure constant.
3. In the fragment shader discard if not u < f < v.

Issues

• Need to compute (u, v) from (p, q, p', q')
• Quite wasteful; usually we will show full segments, but have added 4 floats to each point

See: NeuroML/NeuroML2#144

[thorstenhater]

The trivial case where u = 0 or v = 0 is fixed in master.