ENH: Dont constrain fiducial clicks to mesh vertices

[larsoner]

On main, when you set fiducial positions using clicks, they are forced to be at one of the vertex positions on a mesh. For coarse/smooth head meshes, this unnecessarily limits the precision, sometimes causing locations to move by several millimeters from where the user clicks. This allows the actually-clicked point to be set as the fiducial location, which should work better / more precisely.

Using this on main and cilcking Lock fiducials, then unclicking Show high-resolution head, then unclicking Lock fiducials:

mne coreg --subject sample --subjects-dir ~/mne_data/MNE-sample-data/subjects 

This click is off by a lot as it snaps to the nearest vertex:

On this PR it lands right where I click, wherever I click:

This isn't a big issue for things like sample that have a good high-res head, but for some templates like our infant ones where only a low-res head is available, the points move a lot like this as well.

[larsoner]

Had to do some SSH key gymnastics to get CircleCI to checkout properly. Not sure why, maybe some deploy key expired. One was a personal key from 2016... 🤷 Might have a deploy failure tonight, we'll see. But easy enough to iterate a bit as needed.

[larsoner]

The macos-intel test takes a really long time so I'm going to add a slowtest dec to more tests to get the time down a bit