Can't draw thin, crisp lines with pen

[cwillisf]

Expected Behavior

Drawing alternating 1-pixel stripes with the pen should result in crisp stripes.

This is from Scratch 2.0:

Actual Behavior

Drawing alternating 1-pixel stripes with the pen causes the two colors to mix into a solid average color.

This is from the same project running in Scratch 3.0:

Steps to Reproduce

Load project 224109827 and press the green flag or space bar.

Code for this project:

[cwillisf]

I suspect the pen lines are being drawn at the edge of the pixel rather than the center of the pixel. Fixing this might be as simple as adding or subtracting 0.5 or 0.375 to all pen coordinates.

This is discussed in the OpenGL "Red Book" in the "OpenGL Correctness Tips" section of Appendix G.

[thisandagain]

/cc @rschamp @mzgoddard