Support nesting draw calls

[davepagurek]

Right now I think this breaks:

fbo1.draw(() => {
  fbo2.draw(() => {
    // etc
  })
  texture(fbo2.color)
  rect(0, 0, width, height)
})

This is because the draw call binds the main canvas when it finishes: https://github.com/davepagurek/p5.Framebuffer/blob/main/p5.Framebuffer.js#L206

To fix this, it would need to record whatever context is currently bound before it starts, and then re-bind that at the end again instead of binding null (the main canvas.)

As a temporary workaround, this should be fine:

fbo2.draw(() => {
  // etc
})
fbo1.draw(() => {
  texture(fbo2.color)
  rect(0, 0, width, height)
})