Clamp scroll-snap-align: start snap points inside the scroll range.
This change is basically equivalent with what we did for bug 1788029 but for
scroll-snap-align: start cases.
The test case was originally written by Daniel Holbert.
Differential Revision: https://phabricator.services.mozilla.com/D215315
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1905247
gecko-commit: 4734497618a2e6c8c8b5f7e6f2e8343058484793
gecko-reviewers: emilio