uploads: disallow more video formats not supported by all browsers.

Disallow uploading videos with 10-bit color or 4:4:4 chroma subsampling.
Neither of these features are supported by Firefox.

Only 8 such videos have been uploaded to Danbooru:

* https://danbooru.donmai.us/media_assets/3070695 (4:4:4)
* https://danbooru.donmai.us/media_assets/3070697 (4:4:4)
* https://danbooru.donmai.us/media_assets/3292518 (4:4:4)
* https://danbooru.donmai.us/media_assets/3358659 (10-bit)
* https://danbooru.donmai.us/media_assets/3358660 (10-bit)
* https://danbooru.donmai.us/media_assets/3730866 (10-bit)
* https://danbooru.donmai.us/media_assets/5056665 (10-bit)
* https://danbooru.donmai.us/media_assets/5479605 (4:4:4)

Note that Exiftool doesn't output this information, so it's not in the
EXIF metadata. We have to reply on ffprobe at upload time instead.

Followup to #3615.

app/logical/ffmpeg.rb

@@ -83,6 +83,10 @@ class FFmpeg
     frame_count / duration
   end
 
+  def pix_fmt
+    video_stream[:pix_fmt]
+  end
+
   def video_codec
     video_stream[:codec_name]
   end