Add blendMode

[Schmavery]

https://processing.org/reference/blendMode_.html

Not sure how to do this without triggering a flush, but it could allow for some useful visual effects. Being able to subtract a shape from another shape gives a lot of flexibility...

Anecdote: For a while I couldn't figure out how jared's Purple Maze game worked without this feature, but then realized that's why everything was purple...