Backend Selection API

[OutOfTheVoid]

While not currently the case, in the future, we may have multiple backends on some platforms (E.G. xdg-desktop-portal vs xcb/x on Linux), so having some way to choose these at runtime based on support is necessary before this is the case.

At a minimum, Backend information should report:

• Supported features (GPU APIs, Access control details)
• Content Selection Mode (Window Manager vs Programmatic)
• Performance Details (are frames delivered via GPU textures, or do we have to upload them?)