Only allow LM previews in Object mode

Generating lightmap previews in edit mode is nonsense. Also, there is
some issue in blender with changing the image assignment programatically
while in edit mode, so it's best to just disallow this entirely.

korman/operators/op_lightmap.py

@@ -23,6 +23,8 @@ class _LightingOperator:
 
     @classmethod
     def poll(cls, context):
+        if context.mode != "OBJECT":
+            return False
         if context.object is not None:
             return context.scene.render.engine == "PLASMA_GAME"