ugly png plots in 3.1.54

[Francesco Potorti`]

>This should restore the old behavior.

I tried it with "-S640,480" for both PNG and SVG, and without any option
for PNG, and I would say that everything looks good now, thanks.

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Francesco Potortì (ricercatore)        Voice: +39 050 315 3058 (op.2111)
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