exact forward projector

[JeffFessler]

We later might want to try this for exact strip integrals:
https://en.wikipedia.org/wiki/Sutherland%E2%80%93Hodgman_algorithm

see also
https://arxiv.org/abs/1907.10526

and
http://doi.org/10.1109/42.14519
@A lo:88:sal ieee-t-mi 7 4 355-63 dec 1988
S C B Lo
Strip and line path integrals with a square pixel matrix: A unified theory for computational CT projections