Slow curve rendering on HiDPI screens: use a cosmetic pen for curves of width 1 or less #80

Part of DataLab-Platform/DataLab#453 (curve readability).

Symptoms

With high-DPI scaling enabled (Windows at 125 % or 150 %), a curve with a line width of 1.0 is drawn 1.25 or 1.5 device pixels wide. Qt's raster engine then takes its slow path for wide pens: with antialiasing, drawing a noisy 2000-point curve takes ~800 ms at 150 % instead of ~2 to 3 ms with a one-device-pixel pen (Qt 5.15, measured with QPainter.drawPolyline on a QImage).

Proposed change

Curves (and error bars) whose line width is 1 or less use a cosmetic pen, i.e. a pen whose width is expressed in device pixels and is not affected by the device pixel ratio. The stored width (LineStyleParam.width) is unchanged.

A configuration option (plot section, curve/cosmetic_hairline, enabled by default) restores the previous behavior.