Allow allocationSize/copyTo on opaque VideoFrames when converting to RGB

[Djuffin]

Updated VideoFrame.allocationSize and VideoFrame.copyTo in index.src.html to allow the operation to proceed when VideoFrame.format is null, provided that an output format is specified in options.

This enables applications to convert frames from hardware decoders and capture devices to RGB formats, similar to how they can be drawn to a canvas.

Also fixed some pre-existing HTML structure errors (missing closing </div> tags) in index.src.html which were causing build failures with strict preprocessors.

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