Implementation of Polygon drawing functions

[occamsshavingkit]

I added the public functions:
drawPolygon: draw lines between a list of points, connect the last and first points.
fillPolygon: draw lines between a list of points, fill in what is "inside" the shape.
drawRegPolygon: draw a regular polygon at a center point with a given radius and rotation about its center point. fillRegPolygon: fill a regular polygon like above.

and private helper functions:
sign: returns -1, 0, or 1 depending on the sign of its argument
isRelevant: determines if a given line is relevant for coloring a given point.

all to Adafruit_GFX.h and Adafruit_GFX.cpp

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