Add support for black&white ice (3.7.0 feature) similar to already
supported black&white lava: show in inverse video if it uses the
same character as floor (in the ice case; as water in the lava case).
Inverse for monster detection, black&white lava, and now black&white
ice was being done unconditionally but has been changed so that the
user can disable it by toggling the 'use_inverse' run-time option.
[Bug noticed in the process: if you move an inverse video cursor
onto inverse video detected monster/lava/ice (when doing farlook, for
instance), the cursor disappears. I'm not sure how to address that.]
d984b0b87c