add default mimeType if none is specified

[jadeddelta]

this PR closes #3494, providing a default mimeType: "video/webm" for initializeCameraRecorder() and updating the docs to reflect this change

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[jodeleeuw]

@jadeddelta I think we might need to add a codec part to the string. See lookit/lookit-jspsych#127 (comment).

@becky-gilbert do you think we should use something like your function that gets compatible mime types?

[becky-gilbert]

@jodeleeuw @jadeddelta sorry for the slow response. Yes you could definitely use the getCompatibleMimeType function we're using in lookit-jspsych to find a compatible mime type from a list, or write something similar.

Or you could stick with a single default option, it just depends on the breadth of browser/versions you want to support. The important thing is to make sure to specify a codecs value in all of the mime type strings, e.g. use "video/webm;codecs=vp9,opus" rather than just "video/webm".

We're using webm, and "video/webm;codecs=vp9,opus" would work for recent versions of the most popular browsers. But that won't work on Safari. Another good option is mp4: "video/mp4; codecs="avc1.42E01E, mp4a.40.2" (I can't remember why we didn't use this before - I think it wasn't available in Chrome before but looks like it is now?).