fix diagonal crack

[Former user]

On some systems (e.g. Edward's Nvidia card) a lower-left to upper-right diagonal line is seen on all quads, when polygons are also drawn on the same screen (e.g. in the showcase, from the moment the dot field appears). This may be a driver bug triggered by glEnable(GL_POLYGON_SMOOTH). If that is the cause, maybe the best workaround is simply to avoid GL_POLYGON_SMOOTH (its advantage over GL_POLYGON seems marginal anyway, even on systems for which smoothing improves lines and dots).

[Jeremy Hill]

Worked around: .smoothing=1 remains the default value but it now means "attempt to smooth lines and points only", whereas .smoothing=2 or larger value means "attempt to smooth lines, points AND polygons".