update overmap::display_notes to use overmapbuffer

update code that adds AUTO-notes on vertical movement to use overmapbuffer
update overmap::find_note to use overmapbuffer

update overmap::draw to use only global coordiantes and be static

update overmap::draw_overmap to return abslute coordinates
make sure existing overmaps are loaded when requested
Merge continuing overmap handling cleanup.

Conflicts:
	src/computer.cpp
Restore erase call that prevents left over map from staying on screen after exiting map display.