grafx - Extensible Java Graphics Package Code

Adding class TwoPixelLineSanityTest.

Reformatting the main comment.

Revising comment related to line drawing boundary conditions.

Removing the commented out println() statements.

Documenting the findings in error case.

Holy crapola I finally found an error case. It's some sort of error, I don't
know which.

Getting close to coming up with the error case.

Committing some commented-out println() statements in drawLinePosDxDy() which
explains the step-by-step process of pixel selection.

I am really struggling with this test code. Committing what I've got.

Adding a long comment regarding the possibility that an end pixel won't get
turned on.