gles2: don't disable clip planes for GLES2

GLES2 doesn't provide user clip planes (you would have to use a vertex +
fragment shader to achieve the same kind of result) so we make sure not
to call glEnable/Disable with any of the GL_CLIP_PLANE0..3 defines.

http://bugzilla.o-hand.com/show_bug.cgi?id=2177

clutter/cogl/cogl/cogl-clip-stack.c

@@ -603,7 +603,8 @@ _cogl_clip_stack_flush (CoglClipStack *stack,
 
   has_clip_planes = cogl_features_available (COGL_FEATURE_FOUR_CLIP_PLANES);
 
-  disable_clip_planes ();
+  if (has_clip_planes)
+    disable_clip_planes ();
   disable_stencil_buffer ();
 
   /* If the stack is empty then there's nothing else to do */