Returns whether any part of the map (not the entire screen) matches Regex.
Updates the current_level if Dlvl appears to have changed.
Mark tiles that are obviously explored as such.
Things like "a tile with no unknown neighbors".
Checks the screen to see if we're engulfed.
It'll inform the rest of the system about our engulfedness.
Returns 1 if we're engulfed,
0 if not.