fix COLR font check being racy
fix COLR font check being racy

also make sure it doesn't run more than once.
keeping the FF sniffing because missing "extract canvas data" permissions
would still break the check.

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk