Fix jerky animations for the XRender backend

The code controlling timing for the animations
assumes that all painting is finished when the
Scene has finished painting. Since the X protocol
is asyncronous we need to call XSync (and not XFlush)
to make sure all painting has finished.
Tested for more than 2 weeks without issues.

NB: This is the same commit as
d0cf7ff6da4c6f8bcc684dd6378e2af09d36bb06
due to style changes in kwin code I could not cherry-pick
it cleanly

scene_xrender.cpp

@@ -291,11 +291,11 @@ void SceneXrender::flushBuffer(int mask, QRegion damage)
         XFixesSetPictureClipRegion(display(), buffer, 0, 0, None);
         XRenderComposite(display(), PictOpSrc, buffer, None, front, 0, 0, 0, 0, 0, 0, displayWidth(), displayHeight());
         XFixesSetPictureClipRegion(display(), front, 0, 0, None);
-        XFlush(display());
+        XSync(display(), false);
     } else {
         // copy composed buffer to the root window
         XRenderComposite(display(), PictOpSrc, buffer, None, front, 0, 0, 0, 0, 0, 0, displayWidth(), displayHeight());
-        XFlush(display());
+        XSync(display(), false);
     }
 }