Blender Git Loki

Fix own broken rB95b25e7333c4 (crash on any undo op :/).

Issue here was that buttons_texture_context_compute() was getting scene from (button-customized) context,
before the button paths (and hence, context) had been updated. So after an undo, it was getting an invalid
(freed by undo) scene pointer.

Now update BCONTEXT_SCENE path before calling buttons_texture_context_compute().