Add style stack

[tychedelia]

Adds the style stack, which works similarly to the existing transform stack and cleans up our RenderState struct that had been overflowing with style related options. Also adds the combo push/pop operations that also capture the transform state.

[SableRaf]

Thanks, @tychedelia!

I see this implements push()/pop() and pushStyle()/popStyle(). Does it also include pushMatrix()/popMatrix()?

This is one difference between Processing and p5.js: Processing includes three separate pairs of functions; pushMatrix()/popMatrix(), pushStyle()/popStyle(), and the general push()/pop() (which was added later and does both); while p5.js only includes the consolidated push()/pop() functions.

If it doesn't add too much complexity, I think it'd be good to include all three for backwards compatibility.

See: https://processing.org/reference/push_.html