This might be an artistic choice, but round end caps are supposed to be
a "half circle centered at the end point of the line" as documented
here: https://docs.blender.org/manual/en/dev/render/freestyle/
parameter_editor/line_style/strokes.html#caps

They are a squashed half circle instead.

This patch makes this pure half circles [and also fixes the case where
thickness of beginning was used for both beginning and end of the
stroke]