Xwayland delays Present frame submissions

I'm trying to fix the following issue:

• An X11 Vulkan client uses a MAILBOX present mode.
• The Vulkan WSI sends a PresentPixmap request with a zero target-msc.
• Xwayland receives the request (in present_wnmd_pixmap), but doesn't forward it to the parent Wayland compositor. Instead, it waits for a wl_surface.frame callback.
• When the wl_surface.frame callback is done, Xwayland attaches the new buffer and commits the surface.

Here is a visualization of the Xwayland events. The pink vertical lines are vblanks.

This behavior is an issue because it introduces a whole vblank worth of latency. The X11 client just wants to display a frame at the next vblank, but because Xwayland waits for wl_surface.frame`, it ends up being displayed two vblanks later.

Couldn't Xwayland submit the frame as soon as the client issues the PresentPixmap request?

This would require some changes in the present_wnmd code, but it seems doable.

Note, this issue is different from #971 (even if it's related). #971 is about using presentation-time to provide proper feedback to X11 clients. This issue is about the frame submission timing only.