Linux: True/exclusive fullscreen mode

On Windows with a FreeSync/G-Sync monitor and supporting GPU, variable refresh rate works out of the box in (exclusive?) fullscreen mode - and that is a good thing if Low Framerate Compensation works correctly (or the monitor's VRR range is sufficiently low). On Linux, there does not seem to be any way to achieve this.

VRR otherwise works on my Linux system (FreeSync monitor, Vega 64, Manjaro Testing, 5.10 kernel, KDE Plasma 5.20@X11), https://github.com/Nixola/VRRTest and Doom (in both Vulkan and OpenGL modes) run flawlessly.

I have tried both Vulkan implementations for AMD, various outputs/apis and options (opengl-swapinterval=0, vulkan-swap-mode=immediate, x11-bypass-compositor=yes, override-display-fps...), disabling the compositor etc.

It seems that there is simply no support for actual/exclusive fullscreen mode on Linux. I would love for this feature to be added (for instance utilizing Vulkan's VK_EXT_full_screen_exclusive).

Start by tracing mpv's Linux fullscreen handling and Vulkan output path, then compare the requested VK_EXT_full_screen_exclusive behavior with the existing fullscreen and display options listed in the issue. Done means Linux users can request true or exclusive fullscreen and achieve the expected VRR behavior, with the relevant display paths verified.