q5-webgpu: saveCanvas, get, and copy

[quinton-ashley]

In WebGPU the canvas is cleared at the start of each draw pass. There's no built-in way to keep the previous frame and draw a new frame.

I'm thinking of keeping this default behavior, since q5 is geared more towards interactive art anyway.

I'd like to somehow give users the option to draw onto the previous canvas frame, yet currently saveCanvas, get, and `copy aren't implemented in q5 webgpu.

Here's my idea for how to enable continuous drawing to the canvas.

let prevFrame;

function draw() {
  background(prevFrame);
  
  rect(random(width), random(height), 10, 10);
  
  prevFrame = copy();
}