Use swap events to drive repaints

Don't start the composite timer at the end of performPaint() when a
buffer swap is pending and vsync is enabled.  Instead set
m_composeAtSwapCompletion to true so performPaint() gets called again
as soon as the swap completes.

This makes the repaint cycle look like this:

    scene->paint()
    SwapBuffers()
    Process events
          ·
          ·
    Swap completes
    Fetch and reset damage (if applicable)
    scene->paint()
    SwapBuffers()
    Process events
          ·
          ·
    Swap completes
    ...

This results in a noticeable improvement in animation smoothness with
drivers that support GLX_INTEL_swap_event, since we're now able to
consistently render at the monitor refresh rate.

composite.cpp

@@ -663,7 +663,11 @@ void Compositor::performCompositing()
     // is called the next time. If there would be nothing pending, it will not restart the timer and
     // scheduleRepaint() would restart it again somewhen later, called from functions that
     // would again add something pending.
-    scheduleRepaint();
+    if (m_bufferSwapPending && m_scene->syncsToVBlank()) {
+        m_composeAtSwapCompletion = true;
+    } else {
+        scheduleRepaint();
+    }
 }
 
 bool Compositor::windowRepaintsPending() const