grafx - Extensible Java Graphics Package Code

In line drawing subroutine, renaming 'x' to 'xx' and 'y' to 'yy' because those
are easier to grep for.

Renaming 'd' to 'dd' in line drawing routine because it's easier to grep for.

Adding a little TODO comment.

Computing dzdy and dzdx in one of the line drawing subroutines, thorough
explanation of formula derivations in long comment. Wow, these formulas are
very simple. We need to handle singularities, still. That is TODO.

Moving the computations of dxdy & dydx in line drawing routine down further,
in case there is an early exit.

Taking notes.

Making some simplifications in line drawer.

Changing the order of some operations. The code was examined closely and it
was determined that this has no impact on the algorithm.

Fleshing out more of the line drawing.

Changing order of some operations, after careful examination this should not
have any impact on algorithm correctness.