Replace screenshot in README.md

[VoxelTek]

The screenshot previously featured at the top of the README.md was that of the old Qt-based UI. This new image is of the modern SDL2-based UI, and more accurately reflects how Vanilla currently appears.

One issue with this is that Vanilla no longer displays any UI elements while showing the gamepad video stream. As such, creating a single screenshot that encapsulates both the new UI and the primary functionality of Vanilla is not really possible.

Some alternative screenshots that could be used:

Alternatively, the README.md could show two screenshots side-by-side, one of the Vanilla UI and another of gameplay, such as this:

Another factor to consider is whether to show a desktop screenshot, as Vanilla is now intended primarily for portable platforms such as Android, Nintendo Switch, or the Steam Deck, and is largely intended to run full-screen.

[itsmattkc]

IMO the only screenshot that really makes sense is the Mario Kart 8 one. Seeing Vanilla's menu/UI isn't really that useful for new users, whereas seeing what Vanilla does (i.e. makes a gamepad on your desktop) is.

[VoxelTek]

I've changed it, now it should display the image of Mario Kart 8 running in Vanilla.