Windows: Fixed FileNotFoundError when copying dependencies to the project folder

[Ati1707]

If you copy the dll files to the project folder. It will throw a FileNotFoundError because of the default CDLL mode(LOAD_LIBRARY_SEARCH_DEFAULT_DIRS).
It expects a full path to the file but for some reason ctypes.util.find_library only returns the file name if its in the project directory next to the mpv.py. Thats why the workaround(#60 (comment)) putting the dependencies in the script folder works since it returns the full path.
Adding winmode=0 fixes this issue and the user can now put the dependencies in the project directory or in the script folder.
For unit tests you would need to copy the dependencies in the tests folder as well.

It might be better to use a different approach to find the dll but this might be enough since it shouldn't break anything if it does let me know.

Closes #104

[jaseg]

Adding winmode=0 re-enables searching the CWD, which I think got removed for a good reason.

The underlying issue seems to be that ctypes.find_library searches through %PATH% using string substitutions. When all entries in %PATH% are absolute paths, that will work since passing an absolute path to CDLL later will override the search order anyway. However, when find_library finds the DLL based on a relative path in %PATH%, that will cause the subsequent call to CDLL to choke when the resolve path that DLL is under is not one searched by LOAD_LIBRARY_SEARCH_DEFAULT_DIRS.

After relying on %PATH% so far, I don't really want to change that behavior. We could ad-hoc register any relative path returned by find_library using os.add_dll_directory, but that would affect the whole process, which I'm not a huge fan of. I think for now, the right fix is to just adjust the error message, and tell the user to only ever use absolute paths in %PATH%, combined with calling CDLL with LOAD_LIBRARY_SEARCH_DEFAULT_DIRS | LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR to make sure any possible dependency DLLs of libmpv also get loaded (not sure if there are any on Windows atm).

[jaseg]

I've just pushed a change to main that improves the error messages and adds the new flags to the CDLL call. Please see if that does what you want, and feel free to re-open the PR if it doesn't.