Fractional scaling support

[fossfreedom]

          Fractional scaling is currently implemented using mutter experimental feature key values added as part of a mutter patch on ubuntu and manjaro.

https://github.com/BuddiesOfBudgie/budgie-control-center/blob/953c93dbe5759747d31e9610b66cc09c90815737/panels/display/cc-display-config.c#L20

Other distros obviously don't patch mutter with these. So need to decide the best way forward how to support fractional scaling preferably with least risk, least number of changes required.

Originally posted by @fossfreedom in #1 (comment)