improve scmap color coding to tile function

improve scmap color coding to tile function

Figure out some specific bitmask values of Data::StarCraft::Tileset
->tileavg->{walk} (besides for &1 for obvious walkability, and ignoring
anything above the random pick of 10).

Ramps (height transfers) are always &16 at some point, so mark those
with green because they're usually very significant control points.

&2 and 4 seem to indicate ground level.  Not highlighted yet because
tile color usually gives a good clue already.  Maybe a special mode
later.

&8 appears to be high level, but never for ground, but only sole objects
like statues (blocking vision?). Don't know what to do with those
exactly, but set a bright red color for now to identify them.