- Fix selection not aborting when the preferred provider returns null
- Fix the default video provider being "ffmpegsource" (lowercase)
instead of "FFmpegSource", which would trip up provider selection
- More generally, make sure the preferred video provider actually exists
and fall back to the default (FFmpegSource) if not.
Fixesarch1t3cht/Aegisub#23 .
Fixesarch1t3cht/Aegisub#61 .
Fixesarch1t3cht/Aegisub#83 .
This became necessary now that more providers were added. Providers can
be proritized for certain file types (e.g. .vpy files will always be
opened with VapourSynth), and when the default provider fails on a file,
the user will be notified and be asked to pick an alternative provider.