Floor the MIND variance so a constant batch yields a flat descriptor

The variance MIND divides by was clamped between 0.001 and 1000 times its mean over the
batch. A batch of constant patches, as the online mode of the ITK metric sends over air,
has a mean of 0, so both bounds were 0 and the division gave NaN. A floor of 1e-6 is the
lower bound a whole image of mean variance 1e-3 already imposes on its flat patches; such
a batch now yields exp(0) = 1 on every channel. Textured batches are bit-identical to the
previous export, the floor never binds there.

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