Demonstrates different Porter/Duff compositing modes.
This implementation is purely in software and is slow and should be used for educational purposes only.
See also:
https://www.w3.org/TR/compositing-1/
http://ssp.impulsetrain.com/porterduff.html
https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Compositing
Clear
SourceOnly
DestinationOnly
SourceOver
DestinationOver
SourceIn
DestinationIn
SourceOut
DestinationOut
SourceAtop
DestinationAtop
Xor