All its members are optional, so we can just use it as a plain struct
rather than Maybe<> all around, which simplifies the code and prevents
silly bugs like bug 1779592.
Mostly automatic via:
rg -l 'SVGImageContext' . | xargs sed -i 's/Maybe<SVGImageContext>/SVGImageContext/g'
With trivial build fixes.
Not intended to change behavior.
Differential Revision: https://phabricator.services.mozilla.com/D151846
