Don't move render filetype dropdown if user types a different extension

The render dialog has a selected filetype ("filter") dropdown to pick
what format to save to by default. This PR changes the code to remember
the selected filetype between renders.

Previously we changed the selected filetype if the user types an
extension belonging to a different type in the list, even if they didn't
pick an extension in the list. This can be confusing, so I removed the
code responsible (instead only remembering the most recently selected
list item).

corrscope/gui/history_file_dlg.py

@@ -150,12 +150,6 @@ def get_save_file_path(
 
     user_suffix = Path(out_name).suffix
 
-    # If user explicitly types a different extension, use it next time (even if they
-    # don't change the dropdown). This works on Windows and FIXME Linux? Mac?
-    # FIXME it seems confusing to pick a different default extension if you type but don't dropdown? IDK.
-    if user_suffix and user_suffix in filters:
-        out_suffix = user_suffix
-
     # Append suffix if missing in user-specified name but present in dialog.
     # getSaveFileName() does not automatically append a suffix on Linux KDE.
     if user_suffix == "" and out_suffix: