FIX changed order of specifications -ss befor -i for ffmpeg_extract_subclip()

[grszkthfr]

Refers to #847

• If this is a bugfix, I have provided code that clearly demonstrates the problem and that works when used with this PR
• I have added a test to the test suite, if necessary
• I have properly documented new or changed features in the documention, or the docstrings
• I have properly documented unusual changes to the code in the comments around it
• I have made note of any breaking/backwards incompatible changes

[gpotter2]

Hi ! It seems I was late on this issue 😄 you were here before me and my #882

Here is some doc I could gather, showing that this fix is the correct one:

The fix (I tested it multiple times with various parameters) is directly inspired from https://stackoverflow.com/a/33188399/5459467 which quotes the above doc.

This PR fixes #508