fixes special case where a channel tile is specified over a regular dig
tile. this allows dig-now to produce a flat floor in that case, which is
likely what is intended.
found this bug with blueprint-generated blueprints. if both a channel
and the resulting ramp are explicitly marked in the blueprint (like the
blueprint plugin does), the channel is processed first, pre-creating the
ramp in the tile designated for a ramp. Then, when the ramp designation
is processed, the ramp is already there, which is an invalid tile to
make a ramp on, so the designation is skipped (and therefore not
cleared). this change clears the designation for both the ramp tile and
the channel tile when either is processed. this opens another edge case
where the designation under a channel is a regular 'd' mine, which will
now get ignored and leave a ramp insead of a flat floor. but I'll
address that in the next commit.
we normally take care of any ramp tops above the channeled tile when we dig the ramp below the channeled
tile, but that logic might not run if we channel down into empty space
(or undiggable tiles)
also fix missing dug_tile indices when channels are dug
also fix second boulder/gem being generated with a potentially incorrect
probability when digging channels and ramps
myk002