Fix excessive transcoding time issue

[jitingcn]

add -vn to parameter in order to bypass video during transcoding to avoid excessive transcoding time

[jitingcn]

Before patch

Convert to "tmpo7qdsgsv.flac" for API.
ffmpeg.exe -hide_banner -y -i ".\Rabi-Ribi.mp4" -ac 1 -ar 44100 "tmpo7qdsgsv.flac"
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '.\Rabi-Ribi10.14.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41
    creation_time   : 2019-11-15T13:10:44.000000Z
  Duration: 00:12:25.88, start: 0.000000, bitrate: 3191 kb/s
    Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, bt709), 1280x720, 2998 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      creation_time   : 2019-11-15T13:10:44.000000Z
      handler_name    : ?Mainconcept Video Media Handler
      encoder         : AVC Coding
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 189 kb/s (default)
    Metadata:
      creation_time   : 2019-11-15T13:10:44.000000Z
      handler_name    : #Mainconcept MP4 Sound Media Handler
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> png (native))
  Stream #0:1 -> #0:1 (aac (native) -> flac (native))
Press [q] to stop, [?] for help
[flac @ 000001ac6d8ad2c0] encoding as 24 bits-per-sample
[flac @ 000001ac6d8b00c0] Video stream #0 is not an attached picture. Ignoring
Output #0, flac, to 'tmpo7qdsgsv.flac':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41
    encoder         : Lavf58.29.100
    Stream #0:0(eng): Video: png, rgb24(progressive), 1280x720, q=2-31, 200 kb/s, 29.97 fps, 29.97 tbn, 29.97 tbc (default)
    Metadata:
      creation_time   : 2019-11-15T13:10:44.000000Z
      handler_name    : ?Mainconcept Video Media Handler
      encoder         : Lavc58.54.100 png
    Stream #0:1(eng): Audio: flac, 44100 Hz, mono, s32 (24 bit), 128 kb/s (default)
    Metadata:
      creation_time   : 2019-11-15T13:10:44.000000Z
      handler_name    : #Mainconcept MP4 Sound Media Handler
      encoder         : Lavc58.54.100 flac
frame= 5322 fps= 18 q=-0.0 Lsize=   14629kB time=00:02:57.84 bitrate= 673.9kbits/s speed=0.593x
video:4670046kB audio:14621kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

Interrupted, speed=0.593x that is too sloooooow

After patch

Convert to "tmp2l6eluzc.flac" for API.
ffmpeg -hide_banner -y -i .\Rabi-Ribi.mp4 -vn -ac 1 -ar 44100 tmp2l6eluzc.flac
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '.\Rabi-Ribi.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41
    creation_time   : 2019-11-15T13:10:44.000000Z
  Duration: 00:12:25.88, start: 0.000000, bitrate: 3191 kb/s
    Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, bt709), 1280x720, 2998 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      creation_time   : 2019-11-15T13:10:44.000000Z
      handler_name    : ?Mainconcept Video Media Handler
      encoder         : AVC Coding
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 189 kb/s (default)
    Metadata:
      creation_time   : 2019-11-15T13:10:44.000000Z
      handler_name    : #Mainconcept MP4 Sound Media Handler
Stream mapping:
  Stream #0:1 -> #0:0 (aac (native) -> flac (native))
Press [q] to stop, [?] for help
[flac @ 00000242cc8c0500] encoding as 24 bits-per-sample
Output #0, flac, to 'tmp2l6eluzc.flac':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41
    encoder         : Lavf58.29.100
    Stream #0:0(eng): Audio: flac, 44100 Hz, mono, s32 (24 bit), 128 kb/s (default)
    Metadata:
      creation_time   : 2019-11-15T13:10:44.000000Z
      handler_name    : #Mainconcept MP4 Sound Media Handler
      encoder         : Lavc58.54.100 flac
size=   58686kB time=00:12:25.87 bitrate= 644.5kbits/s speed= 373x
video:0kB audio:58678kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.013908%

speed= 373x
Note the Stream mapping now only mapping the audio:
Stream #0:1 -> #0:0 (aac (native) -> flac (native))