Add missing check as the context override dict may have been copied
since it was assigned, also initialize the context manager with
PyType_Ready, while it didn't cause any errors - it's expected
that all types are initialized.
Support a way to temporarily override the context from Python.
- Added method `Context.temp_override` context manager.
- Special support for windowing variables "window", "area" and "region",
other context members such as "active_object".
- Nesting context overrides is supported.
- Previous windowing members are restored when the context exists unless
they have been removed.
- Overriding context members by passing a dictionary into operators in
`bpy.ops` has been deprecated and warns when used.
This allows the window in a newly loaded file to be used, see: T92464
Reviewed by: mont29
Ref D13126
