A call to `transcoder_exe (unused, unused)` passes the same
string reference for `ffmpeg` and `ffprobe`. If this happens
during the first call of this function, the path for the latter
is cached for both external tools.
This significantly speeds up loading export formats that use ffmpeg.
A single call to ::transcoder_exe() took 300-400ms on Windows.
With multiple formats using an external transcoder, showing the
export dialog could take to 2-3 sec.
