Fix import of avs-files saved in non UTF-8 code page

7ab4c928 changed the x264 CLI to use the UTF-8 code page on Windows 10 which also enabled the use of Unicode file names in AviSynth scripts as long as such scripts are saved as UTF-8. On the flip side this caused old AviSynth scripts saved using legacy code page encodings containing non-ASCII characters to stop working. Third party tools that currently generates AviSynth scripts using legacy code pages are also affected. In order to use such scripts they currently need to be opened with a text editor and saved as UTF-8. Revert this regression.

Pros: AviSynth scripts using legacy code page encodings containing non-ASCII characters will work again.

Cons: AviSynth scripts using Unicode (UTF-8) containing non-ASCII characters will stop working. File names containing Unicode characters outside the active code page can no longer be accessed from AviSynth scripts.

AviSynth scripts containing only ASCII characters are unaffected.