Cycles: Add a UI warning, in case the experimental GPU kernel is used.

The experimental kernel is slower and can cause issues on some cards still, so better communicate it well.

intern/cycles/blender/addon/ui.py

@@ -1358,6 +1358,9 @@ def draw_device(self, context):
         if engine.with_osl() and use_cpu(context):
             layout.prop(cscene, "shading_system")
             
+        if device_type == 'CUDA' and cscene.feature_set == 'EXPERIMENTAL':
+            layout.label(text="Using experimental GPU kernel", icon='ERROR')
+
 
 def draw_pause(self, context):
     layout = self.layout