UMVE: make handles for tone mapping range smaller
UMVE: fix crash on negative values in tone mapping

This also changes the scaling of the histogram to
bin = (num_bins - 1) * log10(1 + 9 * (v - v_min) / (v_max - v_min))
so that v_min gets into the leftmost bin and v_max into the rightmost.
UMVE: allow ignoring zero values for histogram