Consider relaxing the restriction to not paint video controls or captions

[progers]

We're going to add video controls and captions to https://github.com/WICG/html-in-canvas#read-back-allowed-rendering, which is more restrictive than the current behavior of drawing controls and captions without revealing user preferences (e.g., caption customizations).

I think we should revisit this decision for two reasons:

1. Developers will work around it using libraries like video.js which re-implement controls and captions without user preferences.
2. The value of captions without customizations is still very high. For example, youtube does not support respecting OS preferences for caption customizations. By not showing captions by default, we risk developers not supporting captions at all.

@foolip