Workspaces: remove workspace engine, use 3D viewport draw mode instead.

ViewRender was removed, which means we can't get the render engine for files
saved in 2.8. We assume that any files saved in 2.8 were intended to use Eevee
and set the engine to that.

A fix included with this is that .blend thumbails now draw with Clay mode,
and never Eevee or Cycles. These were drawn with solid mode in 2.7, and should
be very fast and not e.g. load heavy image textures.

Differential Revision: https://developer.blender.org/D3156

release/scripts/startup/bl_ui/properties_physics_common.py

@@ -30,8 +30,7 @@ class PhysicButtonsPanel:
 
     @classmethod
     def poll(cls, context):
-        view_render = context.scene.view_render
-        return (context.object) and view_render.engine in cls.COMPAT_ENGINES
+        return (context.object) and context.engine in cls.COMPAT_ENGINES
 
 
 def physics_add(self, layout, md, name, type, typeicon, toggles):