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toggle mask type: "mskn:" vs. "mskup:" (native vs. upsamp)
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keyboard equiv: <none>

This entry has two different states, "mskn:"
(*native* resolution mask stats) and "mskup:"
(registered and *upsampled* to structural
resolution mask stat).

On startup, the default stat is "mskn:", which is
used to (re-)read ("R" button) native-resolution
stats (bfloat or float BRIK format), register
them to the structural underlay, and upsample to
structural resolution.

Clicking on the label "mskn:" (bold to indicate
it is button-like) swaps in the "mskup:" entry,
which is used to read ("R" button) *and* write
("W" button) already registered and upsampled to
structural (1mm^3 or 0.5mm^3) resolution stats
(float BRIKs only).  Note that these are
considerably larger files than the native stats
(64M for 1mm^3), but compress well.

Clicking again toggles back to "mskn:"
