ffmpeg_opt: use get_preset_file_2 for -{v,a,s}pre parsing

-fpre support incoming.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
ffmpeg_opt: get_preset_file_2: handle -fpre

Code from get_preset_file().

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
ffmpeg_opt: reindent

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
ffmpeg_opt: get_preset_file_2: warn if .avpreset is used

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
ffmpeg_opt: get_preset_file_2: support Windows-style preset dir

Code from get_preset_file().

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
ffmpeg_opt: map -pre to opt_preset()

Signed-off-by: Timothy Gu <timothygu99@gmail.com>