Do not fall back to XRender due to XRender not doing very well.

svn path=/trunk/KDE/kdebase/workspace/; revision=756727

NOTES_4_0

@@ -16,6 +16,13 @@ This document is for creating the 4.0 release notes. There are 3 parts:
   if the negative impact becomes insignificant. See section "OPENGL YIELD BEHAVIOR"
   in README.txt for nVidia cards.
 
+- XRender mode: It is possible to use XRender for compositing instead of the default OpenGL.
+  XRender mode in general has less features, however at the moment it is also considered
+  unstable - it has not received as much testing as OpenGL mode, some features may be incomplete
+  and it is recommended to use the OpenGL mode if possible. Also note that current XRender
+  implementations often perform rather poorly.
+
+
 Comments
 ========
 

composite.cpp

@@ -128,7 +128,7 @@ void Workspace::setupCompositing()
                 break; // -->
             delete scene;
             scene = NULL;
-            // fall through, try XRender
+            break; // do not fall back to XRender for now, maybe in the future
 #endif
 #ifdef KWIN_HAVE_XRENDER_COMPOSITING
         case XRenderCompositing: