Workaround for NRE on linux

Fixes #2416
Better way to disable SDL2 backend

See https://github.com/Mono-Game/MonoGame.Dependencies/pull/9
Disable SDL2 backend on linux by default

This can still be enabled explicitly for testing. Closes #2416
Merge pull request #2419 from thefiddler/issue2416

Workaround for NRE on linux