…AX_BLUR after scale factors are applied r=gfx-reviewers,nical

This was causing one of the large drop-shadow wrench reftests to timeout.
This is only a partial fix, as we should be checking the scale factors earlier on when sanitizing the
filter input. This will ensure we match what the non-WR backend is doing and will prevent overinflation.

Differential Revision: https://phabricator.services.mozilla.com/D64197

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/b7ab4c947688fab1ee8de2ef13a6033e2307d590