Only SVG can actually handle transparency, but we should at least mimic
the color value of other transparent fills by blending with a (presumed)
white paper.
Fixes https://gitlab.com/kicad/code/kicad/issues/11304
Also fixes some plot bugs with arcs.
Also moves polygonization of arcs (when required) in plotting code
from 5 degrees to calculated based on ARC_HIGH_DEF.
Fixes https://gitlab.com/kicad/code/kicad/issues/5017
Use 4 digits in mantissa as default and when exporting SVG.
Allow 0 as line width: this is the right value to draw filled shapes with no outline thickness.