DC: Workaround for usleep prototype mismatch
DC: Silenced some stupid warnings
GRAPHICS: Fix computation of addA in darkenFill()

The old computation had rounding issues, causing alpha to leak into
the red (usually) component.  There's a much easier way to compute it
that does not lead to such problems:  What should really happen is that
the two top bits of the A component should be set to 1 (thus adding
75% alpha).  So compute it that way for speed and precision.