Wait until video is ready before capture

[andsouto]

With current code when the polyfill is in used the next code will fail because videoElement.videoWidth will still be 0:

let imageCapture = ImageCapture(track)
imageCapture.takePhoto();

The changes in this PR explicitly waits until the video element is ready waiting for the playing event before capturing the image. This way it works the same way as the native implementation where the previous code will work.

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[andsouto]

I signed it!

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CLAs look good, thanks!

[dandv]

Thanks for the PR! Let's see if this fixes #4 for @RandomUsername07.