add support for additional unit points (pt) for --autofit and -–geometry

it would be nice if we could add another unit points, besides percentage, to the autofit and geometry option. currently everything is unitless and interpreted as pixels. it would be helpful for high hidpi resolutions. currently --hidpi-window-scale decides how the unitless values are being treated. making those option a bit more explicit will avoid problems like #4218.

to avoid backwards incompatibility, anything unitless should be treated as pixel. additionally i don't think pixels and points should be mixed within the option. so if mixed either only the first unit should be treated as right or we only add the unit as a suffix at the end.

--geometry=1920ptx1080+100+100 << would treat everything as points
--geometry=1920x1080pt+100pt+100 << would treat everything as pixel
--geometry=1920x1080+100+100pt << or as suffix, everything points
--geometry=1920x1080pt <<same as above

that way we could probably also deprecated the --hidpi-window-scale option.

Start by locating the command-line option parsing and handling for --autofit, --geometry, and --hidpi-window-scale. Review the unit and mixed-unit examples in the issue; the work is done when point units are supported without changing the meaning of unitless values, with the mixed-unit behavior decided and covered by tests.