conform: Conditionally execute the texture-fbo unit

If we don't have support for offscreen buffers, then there's no point in
testing FBO support in ClutterTexture — a feature that has been long
since deprecated, on a deprecated class.

tests/conform/texture-fbo.c

@@ -176,6 +176,9 @@ texture_fbo (TestConformSimpleFixture *fixture,
   ClutterActor *actor;
   int ypos = 0;
 
+  if (!cogl_features_available (COGL_FEATURE_OFFSCREEN))
+    return;
+
   state.frame = 0;
 
   state.stage = clutter_stage_new ();