Currently (since #12459), when a colorbar is present, the cursor data under imshow() is
formatted using the colorbar's cursor formatter (the idea being that
that formatter should be able to "smartly" take normalization limits
into account); if no colorbar is present a fixed format string ("%0.3g")
is used (see #12473 for a stalled attempt to work around that).

In fact, there is a better scale that defines the number of significant
digits one should display in an imshow cursor data: it arises because
colormaps are discrete (usually with 256 colors, but in any case the
value is available as cmap.N). This quantization tells us, for a
given value, by how much one needs to move before the underlying color
changes (at all); that step size can be used to to determine a number
of significant digits to display. (Even if that's not necessarily
always the best number, it should at least be reasonable given the
user's choice of normalization.)

Also, note that because ScalarFormatter has now changed to take pixel
size into account when determining its number of significant digits (#16776),
the previous approach of relying on the colorbar formatter has become
a less good approximation, as that means that the number of digits
displayed for an imshow() cursor could depend on the physical size of
the associated colorbar (if present).

Also factor out and reuse some logic to compute the number of
significant digits to use in format strings for given value/error pairs,
already used by the linear and polar tickers.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).