Feature: Point (pt) as new space unit for labels

[anne-gropler]

Currently, there is space unit support for pixels (px) and the abstract world unit (world). The former is used for 2D Labels, the latter for 3D Labels.
Since px does not ensure an absolute size (i.e. the visible result depends on the display's dpi), pt would be useful to ensure an absolute size for labels, and thus, ensure labels' readability.

1 pt is 1/72 of an inch based on the physical size of the screen.

See devicePixelRatio.