Add initial prototype of freeform spiral formatting.

The formatter in format/spiral.cu is actually not limited to spiral waveforms only; it uses position data (currently the galvo_target stream, but this can be changed easily to galvo_actual) to "paint" A-scans to the right location in a volume using nearest-neighbor interpolation (though a switch to a linearly interpolated method is possible).

(cherry picked from commit 8481a347)