niivue-display: show location/value and ensure integer location

[chhsiao1981]

We don't want the sub-pixel interpolation in the medical images.
We also need to show the location of the cursor to help the users know where the cursor is pointing at.

[chhsiao1981]

#1390 uses nearest-interpolation.

Able to show primitive location-value info through crosshair.

TODO: better location-value info presentation.