Fixed missing init of ffmpeg-bin locations before calling decklink list.

...but must be invited as collaborator (?) to create a pull request?

I'm using the ffmpeg binaries in `/usr/local/opt/ffmpegdecklink/bin` on my setups.
**It seems that if those are the primary/preferred executables, the shell variables were not initialized when trying to list the decklink devices.**

I've added a call to `_gather_ffmpeg_vars` in `_get_decklink_inputs` to fix that.

I'd also like to propose adding `/opt/ffmpeg/decklink/bin` to vrecord's search paths?
That would be more unix-compliant (/opt), and allow multiple ffmpeg-versions in parallel (as subfolders of /opt/ffmpeg).

Maybe even introduce a plain "PATH"-like variable, replacing the if-then-elif-fi blocks?

Anyways: So I'd like to provide a patch (pull-request). :smiley: