[Video/Wayland] Explicitly waiting for eglSwapBuffers() to complete.

Description

Currently, RetroArch will show noticeable input lag when using the OpenGL backend on the Wayland context (the max_swapchain setting is not available on Wayland + OpenGL).

Buffer swapping is done here for Wayland: https://github.com/libretro/RetroArch/blob/911308327dc1f06531575f9f606b21a0a25ac38a/gfx/drivers_context/wayland_ctx.c#L518
...but it lacks a mean to block until eglSwapBuffers() completes.
eglSwapBuffers() is by default a synchronous/blocking function: in theory it doesn't return until the requested buffer swap is done. This behavior can be changed with eglSwapInterval(1), which make subsequent eglSwapBuffers() calls return immediately.

However, blocking eglSwapBuffers() as things go internally on Wayland means "you can send a new frame", not "the issued buffer swap is complete and the new contents are on screen".
That's why waiting for the buffer swap event "manually" after eglSwapBuffers() can be a good idea.

In the KMS/DRM backend of SDL2, I use events to be notified on the buffer swap completion:
https://github.com/libsdl-org/SDL/blob/5b2884cb0203cc63bf9753f8b55ea4c6c6f19cfb/src/video/kmsdrm/SDL_kmsdrmvideo.c#L391

So, any idea on what would be the equivalent in Wayland for explicitly blocking until the requested buffer swap is completed?

Expected behavior

RetroArch on Wayland using OpenGL should have less input lag.

Actual behavior

RetroArch on Wayland using OpenGL has noticeable input lag.

Steps to reproduce the bug

1. Run RetroArch on Wayland with the OpenGL driver.
2. You get input lag.

Bisect Results

Has always happened.

Version/Commit

Every version or RetroArch has this problem: as of today, there's no mechanism implemented to explicitly wait for vsync after the egl_swap_buffers() call here:
https://github.com/libretro/RetroArch/blob/911308327dc1f06531575f9f606b21a0a25ac38a/gfx/drivers_context/wayland_ctx.c#L514

Environment information

• OS: GNU/Linux with Wayland and any compositor
• Compiler: gcc 10.x, doesn't matter for this.

@Themaister Can you please give me some input here? Are there Wayland-specific mechanisms to force wait for completion after an eglSwapBuffers() call?

Start with gfx/drivers_context/wayland_ctx.c around the eglSwapBuffers() call at lines 514–518, then compare the swap-completion handling in SDL's src/video/kmsdrm/SDL_kmsdrmvideo.c. Determine the appropriate Wayland mechanism and verify that the OpenGL backend explicitly waits for buffer-swap completion and reduces the reported input lag.