Add watch-container-size prop

[weotch]

When true, keeps the container size variables up to date even if there is no video.

In addition, if there is an object image config, the container size will be used for determining which image to show, not the window. Thus, if an image takes only 50% of the width, this will use a smaller image than it normally would.

• Do a JS perf to see if reading only one dimension is faster.

[weotch]

Reading only one axis takes half the work, so I should only update the width when doing this: https://jsperf.com/does-reading-one-offset-improve-performance