gfx: Use unsigned type for bit fields
duck: Add function for testing duck visibility
game: Use duck visibility test

This fixes a bug that occured when running under VirtualBox: VBox does
not emulate the timing of the vertical retrace, which results in a very
high frame rate.  As a consequence, many ducks were getting deleted
immediately after their creation because they had no time to move into
the visible area between two game updates, which the game interpreted as
having moved outside the visual area.
game: Make the rate of duck creation independent of the frame rate
Update Emu.txt