[Fontations-backend] Add default axes to filtered collection

`SkTypeface::getVariationDesignPosition()` expects default axes
to be present and set to the default value. Prepopulate the argument to
Skrifa's AxisCollection::filter() with the default axes and their
values, then let the user specified values override them. This fixes the
behavior of getVariationDesignPosition() (in particular for its return
value = number of axes) also for a situation where no axis coordinate
position was provided in SkFontArguments when creating the SkTypeface().

Test:

Improves test pass rate when running:
    $ dm -m 'TypefaceAxes' --fontations

Bug: skia:328418459
Change-Id: I067f11c9a7e3cd52cb051a6317f3e053276282e8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/827936
Reviewed-by: Dominik Röttsches <drott@google.com>
Commit-Queue: Dominik Röttsches <drott@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed