Prevent high eye encumbrance from blinding players

Due to the way range of vision was calculated, when the player had high
eye encumbrance, it could become negative and prevent the player from
seeing anything, including their own tile.
The player should never be unable to tell where they are, even if they
cannot see anything else, so prevent range from becoming less than 1.

src/character.cpp

@@ -1254,6 +1254,9 @@ float Character::get_vision_threshold( float light_level ) const
         range++;
     }
 
+    // Clamp range to 1+, so that we can always see where we are
+    range = std::max( 1.0f, range );
+
     return std::min( static_cast<float>( LIGHT_AMBIENT_LOW ),
                      threshold_for_range( range ) * dimming_from_light );
 }