Improve player dimension accuracy in certain cases

[jplhomer]

In PR #418, the use of getComputedStyle was replaced with getBoundingClientRect in order to support IE11.

However, this breaks implementations where the video is being scaled using transforms because getBoundingClientRect returns the scaled output, not the original output.

This PR updates the getPlayerWidth and getPlayerHeight methods to use getComputedStyle and fall back to getBoundingClientRect when necessary. This allows us to support scaled videos, normal videos, and fluid sizing on IE11.

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