[ptx] Projections (was Alternative fisheye projections)

[Peter Gawthrop]

I'd like to play around with other projections.  The idea is to
stitch a rectilinear 2x1 and then project it to make it look good.

So far, I have modified sphere-slicer.pl by Bruno Postle to do this
for Mercator's projection (using formulae from
http://mathworld.wolfram.com/topics/MapProjections.html). The problem
is that it is slow. Before I plunge in to other projections, I would
like to be sure I am on the right path.

Do you thing perl is the correct approach? Or would modifying nona/hugin be
a better approach? I guess the main issues for me are speed and
interpolation quality.

Any ideas welcome,

    Peter.