Vulkan driver can't do fractional scaling in Wayland mode and there is no X11 env var override

I'll be brief as to not waste your time:

• The Vulkan video driver does not have support for fractional DPI scaling in native Wayland mode. Therefore, the image looks blurry and the window size is wrong (scaled up)
• This does not happen if Retroarch runs in xWayland mode, where the window size is correct and the game is not blurry (at least under KDE Plasma)
• However, it is impossible to run Retroarch with the Vulkan driver in xWayland mode because currently it just unconditionally uses Wayland on Wayland sessions with no possibility of override. You can force the sdl2 driver to run in xWayland with SDL_VIDEODRIVER=x11 but there's no such thing for the Vulkan driver.

So my request is, of course, that fractional scaling is implemented eventually. But regardless of that there should really be an env var to force the Vulkan driver to operate in X11 mode. Sometimes xWayland is a superior experience to native Wayland and right now Retroarch is such a case.

Thanks.

Start by locating the Vulkan video driver and the existing SDL2 handling for SDL_VIDEODRIVER. Compare how Wayland and X11 are selected during Wayland sessions, then define whether the work covers only an X11 override or also fractional scaling; done means the selected mode produces the expected window size and image quality.