Adding vector graphics; add `atan2` as well. Fixed function caching.

[ldemailly]

using golang.org/x/image/vector

adding

image.move_to image.line_to image.close_path image.draw

sample generates

difficult part is/was what to do with alpha when adding

[ldemailly]

With the line with thickness demo:

[ldemailly]

examples/disc.gr:

[ldemailly]

circle: thickness 40

thickness 1

[ldemailly]

Random bezier points (discord.go version)