Fix flickering/blinking IMSC Subtitles

[mattjuggins]

This is detailed further in issue #4358.

This is a proposed fix to prevent subtitle flickering, where the time between removal and appending of divs representing subtitle cues can have gaps in between. Using the stream in the reference player titled “[BBC] On-demand Elephant's Dream - with EBU-TT-D 'Snaking' Subtitle Track (random text) - lines grow over time simulating live subtitles”, it’s clear to see the effect before and after this code is applied.

These changes preserve the work done by @bbert in #4103 & #4155. Extending identical cues is still preserved and I have checked the test stream mentioned in the PR still operates as expected. Additionally, this PR doesn’t affect the operation of WebVTT subtitles or CEA608 captions. The intent is to just target imsc subtitles where this flickering effect has been witnessed.

The following changes have been made:

• Clear up the captionsContainer immediately before adding new elements in _renderCaption().
• Use requestAnimationFrame (where available) for performance benefits and to ensure drawing as a cause of _renderCaption() happens in the same frame (it should if not available anyway now).
• If cues are adjacent, and not identical, make sure they are cleaned up within _renderCaption() and not at a different time by the onexit event. Ensure this only happens for imsc subs.
• Refactor around _extendLastCue() for clarity.
• Separate out arguments for the imscJS renderHTML function, for readability. I have not changed the arguments, just the formatting.