Extend the Screen interface with a new Tty method, which returns the
underlying tty when the screen is a terminal. This enables direct
drawing to the Tty.
Implement the interface for the three screen implementatinos included in
the library.
Extend the Screen interface with a new LockRegion method. This method
sets or unsets a lock on a region of cells. This will be used in
subsequent commits to enable direct drawing to the underlying TTY. The
locks are necessary in order to prevent cells from being drawn on top of
a directly drawn cell.
Implement this interface for the three screen implementations included
in the library.
Reverse video, not reverse text.
Provide an XTERM equivalent default palette.
Reset colors should go to silver on black.
Black color 0 should be rendered properly.
