HiDPI: Add an option to enable scaling for legacy plugins

Definitely! I’m aware it’s the VST devs who don’t follow the rules here, but it’s impossible to convince hundreds of them - incl. such heavyweights like NI, Waves, Soundtoys, etc. - to update their plugins. Instead, other DAWs like Bitwig, Live, Studio One and even Reason handle this themselves, by scaling plugins that don’t support high-DPI.

Please fix this!