WIP: Visuals tests

Currently basic tests for different types of polygons, ellipses and arcs have been added.
Hi, I am getting some weird bugs with testing using TestingCanvas. 
When I use https://gist.github.com/mssurajkaiga/c90f0b2f8ecb144c343c , there is a size mismatch in all the screenshots. The ```viewport``` defaults to (600, 800) even though it shouldn't.
When I use ```assert_image_equal``` as a method of ```TestingCanvas```, the viewport issue vanishes but the screenshots are garbage values or black screens.
I'm investigating it further but wanted to let you know in case you happen to know the cause.