Clean up DOM when using `container` option upon re-running or Cycle `.dispose()` being called

When the Cycle instance running this driver calls .dispose() (e.g. in response to cycle-restart receiving a HMR update) any generated elements/objects should be cleaned up, this already happens implicitly when using the fullscreen automatic element or the canvas option as Regl directly replaces those elements when running the next time, but if specifying container you end up with multiple canvas elements. This might be the intended behaviour, I'm just opening it as an issue because it's not the behaviour I expected.

For example, if you use something like this and get a HMR update:

makeReglDriver({
  container: document.getElementById('viewport')
}),

canvas elements from previous executions will remain inside the #viewport element, a new canvas with the new result is appended to #viewport.

My guesses for a potential solution would be one of:

When options.container is set:
- Hard: Unset the container's children as an explicit cleanup step, only removing children that were created by this driver, this will require the driver to track the elements regl creates.
- Easy: Clear all of the container element's children out prior to calling setupRegl.

cyclejs-community / cycle-regl

Clean up DOM when using `container` option upon re-running or Cycle `.dispose()` being called #1