backends/wayland: Flush rendering commands in endFrame()

eglSwapBuffers() used to flush rendering commands but now that we render
to dmabufs, we need somehow to flush the rendering commands to the dmabuf.

It fixes some of the weird rendering issues that I see sometimes.

src/backends/wayland/wayland_egl_backend.cpp

@@ -205,6 +205,9 @@ std::optional<OutputLayerBeginFrameInfo> WaylandEglPrimaryLayer::beginFrame()
 
 bool WaylandEglPrimaryLayer::endFrame(const QRegion &renderedRegion, const QRegion &damagedRegion)
 {
+    // Flush rendering commands to the dmabuf.
+    glFlush();
+
     m_damageJournal.add(damagedRegion);
     return true;
 }