This mostly affects physics panels. Any engines relying on
RenderEngine.use_game_engine flag to show/hide panels will need to be
updated. The COMPAT_ENGINES technique is how we usually deal with this.
One issue with use_game_engine is that I cannot find a way to set it; it
appears only the BGE can set it. This means (without this commit)
external RenderEngines cannot get rid of the default physics panels.
The RE_GAME flag (the C flag behind use_game_engine) is pretty hacky
and we should look into removing its usage where possible.
3a4c307652