Feature: support stroke / line thickness

[ssadl...@gmail.com]

Currently to draw a rectangle with a fixed size border is possible by drawing 2 rectangles with the same position, one smaller than the other.

But I couldn't see a way to draw a polygon with a fixed size border, and the library doesn't seem to support line thickness at all.

My use case would be solved with either of the following additional constructors on Picture:

StrokeWidth :: Double -> Picture -> Picture

(setting GL.lineWidth), or:

WrapPicture :: (IO a -> IO a) -> Picture -> Picture

(allowing a custom IO action to wrap the rendering of a picture).

Thanks!