Add "--threads" parameter to x264 codec.

[alvestrand]

This adds a parameter for controlling the number of threads to the x264 codec, removing the previously hardcoded default. NOTE: WILL INVALIDATE STORED ENCODES.

It also adds a new tool "vary_parameter" that lets one run encodes for all possible values of a parameter, so that response curves can be seen.

It also fixes a minor Lint error in openh264.py.

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[alvestrand]

Another email address added to profile and verified.

[alvestrand]

I signed it!

[phoglund]

Assign me if you want this reviewed.

[alvestrand]

Chiefly pushing this to test the CLA-bot. It's awaiting feedback from the x264 developers, since it seems to impact performance negatively along at least 2 axes (PSNR and total amount of CPU used). Review can wait until x264 folks give feedback.

[alvestrand]

Verified with x264 folks that the results I observe is what they expect me to see. Please review.

[phoglund]

Had some minor comments.

[alvestrand]

Updates submitted!

[phoglund]

lgtm