[platforms/wayland] Do an initial xdg-surface commit

Summary:
After setting the xdg-surface role an initial commit must be sent per
protocol description.

This fixes Wayland nested session after commit 84de8d135d made this
requirement explicit.

Test Plan: Launched Wayland nested session.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25127

plugins/platforms/wayland/wayland_output.cpp

@@ -99,6 +99,8 @@ XdgShellOutput::XdgShellOutput(Surface *surface, XdgShell *xdgShell, WaylandBack
         }
         updateWindowTitle();
     });
+
+    surface->commit(Surface::CommitFlag::None);
 }
 
 XdgShellOutput::~XdgShellOutput()