Automatically check preserveDrawingBuffer setting?

[bennlich]

Hey there! I was having an issue where my .webms were filled with black frames, and the fix was to set preserveDrawingBuffer: true on my webgl context.

It took me quite a while to figure out that this was the issue. I'm wondering how to make this easier to debug. It would be cool if ccapture could auto-detect that your context settings were borked. Maybe something like:

capture(canvas) {
  let glContext = canvas.getContext('webgl');
  if (glContext && !glContext.getContextAttributes().preserveDrawingBuffer) {
    console.warn("Your canvas context does not have preserveDrawingBuffer enabled. You might get black frames!");
  }
  ...
}

Or, if that doesn't sound like a good idea, maybe simply add something to the readme?

Let me know what you think. Am happy to open a PR.