Set canvas context attributes

[quinton-ashley]

Motivation:

Users should be able to set canvas context attributes.

p5.js currently doesn't expose the ability to change 2d canvas context attributes. They can only be changed when the canvas is created and getContext is used for the first time.

I'm especially interested in being able to set new defaults for q5.js:

Q5.canvasOptions = {
	alpha: false,
	desynchronized: true,
	colorSpace: 'display-p3'
};

alpha: most sketches do not take advantage of the canvas element's transparency and disabling it would improve performance

desynchronized: most sketches would benefit from desynchronization with the DOM, unless the sketch used DOM elements above the canvas.
https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext
https://developer.chrome.com/blog/desynchronized

colorSpace: If the display supports "display-p3" I want q5 to use it. #21

Related issue:
processing/p5.js#5902

Implementation:

This could be done via a createCanvas input parameter. For example:

createCanvas(800, 600, '2d', {
  alpha: false
}); 

Users could also change the defaults held in Q5.canvasOptions which would have an effect on all instances of q5. I will need to do this on p5play.org because I actually do take advantage of the canvas's alpha via the clear function which makes light and dark mode possible on the site without needing to change the example code.