Pass EGL_PLATFORM=DRM to XWayland

XWayland needs to create an eglDisplay for platform DRM. Depending on
the mesa version eglInitialize expects that the passed native display
is a Wayland display and crashes. This prevents KWin to startup.

This patch sets EGL_PLATFORM for Xwayland. This is actually not KWin's
job, but if it otherwise crashes it's better than nothing.

As Xwayland only supports DRM, it's fine to use this.

main_wayland.cpp

@@ -324,6 +324,7 @@ void ApplicationWayland::startXwaylandServer()
     m_xwaylandProcess->setProgram(QStringLiteral("Xwayland"));
     QProcessEnvironment env = m_environment;
     env.insert("WAYLAND_SOCKET", QByteArray::number(wlfd));
+    env.insert("EGL_PLATFORM", QByteArrayLiteral("DRM"));
     m_xwaylandProcess->setProcessEnvironment(env);
     m_xwaylandProcess->setArguments({QStringLiteral("-displayfd"),
                            QString::number(pipeFds[1]),