title: Flutter_ffmpeg: At least one output file must be specified
body: <p>I'm trying to crop a video with <a href="https://pub.dev/packages/flutter_ffmpeg" rel="nofollow noreferrer">Flutter_ffmpeg</a> package using this <code>-i $inputPath -filter:v "crop=80:60:200:100" -c $outputPath</code>, but I'm having this error message </p>
  
  <pre><code>Running FFmpeg with arguments: [-i, /data/user/0/com.timz/app_flutter/Movies/flutter_test/1584827688309.mp4, -filter:v, crop=80:60:200:100, -c, /data/user/0/com.timz/cache/output.mp4].
  I/flutter (20728): ffmpeg version git-2020-01-25-fd11dd500
  I/flutter (20728):  Copyright (c) 2000-2020 the FFmpeg developers
  I/flutter (20728): 
  I/flutter (20728):   built with Android (5220042 based on r346389c) clang version 8.0.7 (https://android.googlesource.com/toolchain/clang b55f2d4ebfd35bf643d27dbca1bb228957008617) (https://android.googlesource.com/toolchain/llvm 3c393fe7a7e13b0fba4ac75a01aa683d7a5b11cd) (based on LLVM 8.0.7svn)
  I/flutter (20728):   configuration: --cross-prefix=aarch64-linux-android- --sysroot=/files/android-sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/sysroot --prefix=/home/taner/Projects/mobile-ffmpeg/prebuilt/android-arm64/ffmpeg --pkg-config=/usr/bin/pkg-config --enable-version3 --arch=aarch64 --cpu=armv8-a --cc=aarch64-linux-android24-clang --cxx=aarch64-linux-android24-clang++ --target-os=android --enable-neon --enable-asm --enable-inline-asm --enable-cross-compile --enable-pic --enable-jni --enable-optimizations --enable-swscale --enable-shared --disable-v4l2-m2m --disable-outdev=v4l2 --disable-outdev=fbdev --disable-indev=v4l2 --disable-indev=fbdev --enable-small --disable-openssl --disable-xmm-clobber-test --disable-debug --enable-lto --disable-neon-clobber-test --disable-programs --disable-postproc --disable-doc --disable-htmlpages --disable-manpages --disable-podpages --disable-txtpages --disable-static --disable-sndio --disable-schannel --disable-securetransport --disable-xlib --disable-cuda --disable-cuvid --disa
  I/flutter (20728):   libavutil      56. 38.100 / 56. 38.100
  I/flutter (20728):   libavcodec     58. 65.102 / 58. 65.102
  I/flutter (20728):   libavformat    58. 35.101 / 58. 35.101
  I/flutter (20728):   libavdevice    58.  9.103 / 58.  9.103
  I/flutter (20728):   libavfilter     7. 70.101 /  7. 70.101
  I/flutter (20728):   libswscale      5.  6.100 /  5.  6.100
  I/flutter (20728):   libswresample   3.  6.100 /  3.  6.100
  I/flutter (20728): Trailing option(s) found in the command: may be ignored.
  D/flutter-ffmpeg(20728): FFmpeg exited with rc: 1
  I/flutter (20728): Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/data/user/0/com.timz/app_flutter/Movies/flutter_test/1584827688309.mp4':
  I/flutter (20728):   Metadata:
  I/flutter (20728):     major_brand     : 
  I/flutter (20728): mp42
  I/flutter (20728): 
  I/flutter (20728):     minor_version   : 
  I/flutter (20728): 0
  I/flutter (20728): 
  I/flutter (20728):     compatible_brands: 
  I/flutter (20728): isommp42
  I/flutter (20728): 
  I/flutter (20728):     creation_time   : 
  I/flutter (20728): 2020-03-21T21:54:56.000000Z
  I/flutter (20728): 
  I/flutter (20728):     com.android.version: 
  I/flutter (20728): 9
  I/flutter (20728): 
  I/flutter (20728):   Duration: 
  I/flutter (20728): 00:00:07.17
  I/flutter (20728): , start: 
  I/flutter (20728): 0.000000
  I/flutter (20728): , bitrate: 
  I/flutter (20728): 3870 kb/s
  I/flutter (20728): 
  I/flutter (20728):     Stream #0:0
  I/flutter (20728): (eng)
  I/flutter (20728): : Video: h264 (avc1 / 0x31637661), yuv420p(tv, bt709), 720x480, 3854 kb/s
  I/flutter (20728): , SAR 1:1 DAR 3:2
  I/flutter (20728): , 
  I/flutter (20728): 29.44 fps, 
  I/flutter (20728): 29.83 tbr, 
  I/flutter (20728): 90k tbn, 
  I/flutter (20728): 180k tbc
  I/flutter (20728):  (default)
  I/flutter (20728): 
  I/flutter (20728):     Metadata:
  I/flutter (20728):       rotate          : 
  I/flutter (20728): 270
  I/flutter (20728): 
  I/flutter (20728):       creation_time   : 
  I/flutter (20728): 2020-03-21T21:54:56.000000Z
  I/flutter (20728): 
  I/flutter (20728):       handler_name    : 
  I/flutter (20728): VideoHandle
  I/flutter (20728): 
  I/flutter (20728):     Side data:
  I/flutter (20728):       
  I/flutter (20728): displaymatrix: rotation of 90.00 degrees
  I/flutter (20728): 
  I/flutter (20728):     Stream #0:1
  I/flutter (20728): (eng)
  I/flutter (20728): : Audio: aac (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 12 kb/s
  I/flutter (20728):  (default)
  I/flutter (20728): 
  I/flutter (20728):     Metadata:
  I/flutter (20728):       creation_time   : 
  I/flutter (20728): 2020-03-21T21:54:56.000000Z
  I/flutter (20728): 
  I/flutter (20728):       handler_name    : 
  I/flutter (20728): SoundHandle
  I/flutter (20728): 
  I/flutter (20728): At least one output file must be specified
  </code></pre>
  
  <p>I've been cracking my head with this for the past two days, kindly share your thoughts on what might wrong.</p>