"Will this game work?" mode

[wheaney]

Add a mode to the plugin that will allow people to launch a game without glasses connected and see if the game will work with virtual display mode. Would help resolve situations like this.

Two ways to approach this:

1. Have the shader just render some kind of banner image that shows a checkmark or "works!"
2. Allow the Steam Deck's gyro (or just the mouse, which may actually achieve the gyro solution as well) to move the screen.

[cimba007]

I would really appreciate a test mode!