`WM_operator_properties_filesel()` allows C operators to set a display or sort
type for the File Browser to use. But the File Browser would always override
that because of an invalid `_is_set()` check. (The operators don't actually set
the value, they only set the property's default value.)
The only operator affected by this is "Recover Auto Save". It is supposed to
show a vertical list ordered chronologically. It used settings from the
previous File Browser usage before this patch.
Operators using the File Browser should generally use
`FILE_DEFAULTDISPLAY`/`FILE_SORT_DEFAULT` now, except if they have a reason not
to. See comments at their definition.
----
This makes it so operators that set a different display or sort type
don't change the sort or display type for the next File Browser operation.
So using "Recover Auto Save" entirely isolates display and sort type from other
operations.
Differential Revision: https://developer.blender.org/D8598
Reviewed by: Bastien Montagne
Ensure we use the first frame as filepath so we can compute the number of
leading zeros. For file validation, always test the first frame rather than
the current scene frame.
Note this only changes cases where the variable was declared inside
the for loop. To handle it outside as well is a different challenge.
Differential Revision: https://developer.blender.org/D7320
