The _floodfill functions fill an area of the display using the current
color and fill mask.
The _floodfill function begins filling at the view coordinate point (x,
y).
The _floodfill_w function begins filling at the window coordinate point
(wx, wy).
If this point lies inside the figure, the interior is filled; if it lies
outside the figure, the background is filled. The point must be inside or
outside the figure to be filled, not on the figure boundary itself.
Filling occurs in all directions, stopping at the color of boundary.
The _floodfill_w routine is implemented as a macro.
Return Value
The _floodfill functions return a nonzero value if the fill is successful.
It returns 0 if the fill could not be completed, the starting point lies
on the boundary color, or the start point lies outside the clipping
region.