Don't force distance to be an `int`

[RealOrangeOne]

Currently, the distance is required to be an int. Depending on what unit is used for measurements, this isn't always true. Changing to a float not only allows fractional distances, but by extension improves precision.

[PeterJCLaw]

Note that there are quite a few PRs in flight which change a lot of the coordinates stuff, so work on this is likely to conflict with them.

[PeterJCLaw]

A related and possibly desirable change may be to allow the specification of marker sizes as float, so that a more sensible unit can be used (currently consumers tend towards using millimetres due to the int limitation).