Add support for a `fuzzy-range` keyword in reftest.list.
It is similar to `fuzzy` but it allows multiple pairs of
`max_difference, num_differences` numbers that introduce
multiple buckets of allowed differences.

For example, `fuzzy-range(5,100,20,10)` allows at most
100 pixels with a difference of at most 5, _plus_ an extra
10 pixels at most that have a difference more than 5 but
less than or equal to 20.
The total number of differing pixels allowed is thus 110,
but only if 100 of those differ by <= 5 and the remaining
10 by <= 20.
110 pixels with a difference <= 5 will still fail.
This is intentional to encourage tighter bounds in tests
where many pixels are slightly off and a few outliers are
off by a lot.

The number of parameters is arbitrary; longer lists can
get confusing so this change also introduces optional
support for writing `<=` in front of the max difference
and `*` in front of the max pixel count, eg.
`fuzzy-range(<=5,*100,<=20,*10)` (no spaces).

Any pixels that exceed the highest maximum will fail the
test, similar to `fuzzy`.

Steps tested:
1. the same tests fail in exactly the same way before and after;
2. reordered the `fuzzy` statements for raster_root_A/B/C to no longer
   be sorted by max difference, and verified that the tests pass/fail
   the same way;
   (then sort them again which is easier to understand);
3. tests using the new feature still fail when the ref no longer matches
   (deliberately broke the _ref version and verified test failed);

Differential Revision: https://phabricator.services.mozilla.com/D65247

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/8c07488c9a3f0a76e01ce34e060d071a2310612d