Adds straightHypLine for the diameter geodesics, and rearranges so that
HypLine is abstract, with StraightHypLine and CircHypLine extending it.

Signed-off-by: John Lawson <john@jwlawson.co.uk>
Changes draw to not be transparent

Signed-off-by: John Lawson <john@jwlawson.co.uk>
Changes StrHypLine to only have one builder for both segments and
diameters

Signed-off-by: John Lawson <john@jwlawson.co.uk>