Fix default size for points elements

When points elements are being shown, an optimal size for the points is guessed based on a heuristic implemented in _calc_default_radii().

There are three fixes that we need to make here:

• get_extent(), even when using exact=False, is too slow when there is a large number of points. We should add a parameter fast: bool = False which computes a very rough approximation of the extent based on some "Montecarlo" sampling.
• The value in unit we return is wrong when we are in a scaled coordinate system; I think that we need to divide by the average module of the eigenvalues of the affine transformation, as we do in _adjust_radii_of_points_layer()
• There is a strange bug in get_extent() where the single-dispatch doesn't recognize SpatialData objects. This is tracked here: https://github.com/scverse/spatialdata/issues/500

For the moment, I am disabling the use of _calc_default_radii() in https://github.com/scverse/napari-spatialdata/pull/215.

Start with _calc_default_radii() and get_extent(), then compare the unit handling in _adjust_radii_of_points_layer(). Check the linked SpatialData issue and napari-spatialdata PR 215 for context on the single-dispatch problem and the temporary disabling change. Done means the fast extent path, scaled-coordinate sizing, and SpatialData dispatch behavior are corrected.