[mediacapture-main] Pull Request: Define the fitness distance for all required constrains.

eehakkin has just submitted a new pull request for https://github.com/w3c/mediacapture-main:

== Define the fitness distance for all required constrains. ==
This CL defines the fitness distance also for required constrains whose
value contains a member named 'ideal' in addition to members named
'exact', 'max' and/or 'min'.

Lets take a **width** constraint `{ideal: 1920, min: 320}` as an example and lets assume that the browser supports the constraint and that there are settings dictionaries which satisfy the constraint (so fitness distance algorithm steps 1 and 2 can be ignores).
Then,
* the step 3 does not apply because the constraint is required (has 'min'),
* the step 4 does not apply because the ideal value is specified, and
* the steps 5 and 6 do not apply because the constraint is not non-required but required,

thus the fitness distance is undefined.

Clearly, the step 5 should apply and the fitness distance should be positive infinity for settings dictionaries with width less than 320, 0 for settings dictionaries with width of 1920 and  |actual - 1920| / max(|actual|, 1920) for other settings dictionaries.

See https://github.com/w3c/mediacapture-main/pull/722


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