Add method for drawing polygons.
For non-filled polygons, uses the existing line-drawing code to render
arbitrary polygons using the given coords list, at the given x,y position,
in the given colour.
For filled polygons, arbitrary closed polygons are rendered using a fast
point-in-polygon algorithm to determine where the edges of the polygon lie
on each pixel row.
Tests and documentation updates are also included.
Signed-off-by: Mat Booth <mat.booth@gmail.com>
