Weston crashes on launching dolphin-emu on a Nouveau GPU using PRIME

Here is the relevant log from stderr:

[11:12:21.835] Spawned Xwayland server, pid 4179
glamor: No eglstream capable devices found
[11:12:21.962] xfixes version: 5.0
[11:12:21.966] created wm, root 924
The XKEYBOARD keymap compiler (xkbcomp) reports:
> Warning:          Unsupported maximum keycode 569, clipping.
>                   X11 cannot support keycodes above 255.
> Internal error:   Could not resolve keysym Invalid
Errors from xkbcomp are not fatal to the X server
weston: cairo-xcb-screen.c:219: _get_screen_index: Assertion `!"reached"' failed.
(EE) failed to read Wayland events: Broken pipe

Since this is Xwayland, running a Qt application (so otherwise using EGL), this might be caused by drawing the frame around the X11 window.

I’m on Weston master (40c519a3), Xwayland 1.20.8, Mesa 20.1.0, Linux 5.6.15, on a Nvidia 1070 running Nouveau, while Weston renders/scanouts on an Intel UHD630.

This might be linked to xorg/xserver#1035 (closed), which happens in the exact same environment, except without using DRI_PRIME=1.