Add a dropdown to specify Ruffle's render backend

[n0samu]

Is your feature request related to a problem? Please describe.
On my laptop, only Ruffle's OpenGL rendering backend works reliably. But Ruffle doesn't use it by default.

Describe the solution you'd like
Add a dropdown to the launcher's Config tab allowing to specify the render backend (default, Vulkan, DX12, OpenGL or Metal)

Additional context
The old FPL-Ruffle launcher extension had this dropdown.

[colin969]

Done in 4a06334