Fixes these exceptions:
```
[2019-12-22 20:48:14.779 +01:00] [ERR] Error in WaitForExit
System.InvalidOperationException: No process is associated with this object.
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.GetWaitState()
at System.Diagnostics.Process.WaitForExitCore(Int32 milliseconds)
at System.Diagnostics.Process.WaitForExit(Int32 milliseconds)
at Emby.Server.Implementations.Diagnostics.CommonProcess.WaitForExit(Int32 timeMs) in /home/pi/dev/jellyfin/Emby.Server.Implementations/Diagnostics/CommonProcess.cs:line 100
at MediaBrowser.MediaEncoding.Encoder.MediaEncoder.StopProcess(ProcessWrapper process, Int32 waitTimeMs) in /home/pi/dev/jellyfin/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs:line 785
[2019-12-22 20:48:14.790 +01:00] [INF] Killing ffmpeg process
[2019-12-22 20:48:14.795 +01:00] [ERR] Error killing process
System.InvalidOperationException: No process is associated with this object.
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.Kill()
at Emby.Server.Implementations.Diagnostics.CommonProcess.Kill() in /home/pi/dev/jellyfin/Emby.Server.Implementations/Diagnostics/CommonProcess.cs:line 95
at MediaBrowser.MediaEncoding.Encoder.MediaEncoder.StopProcess(ProcessWrapper process, Int32 waitTimeMs) in /home/pi/dev/jellyfin/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs:line 799
[2019-12-22 20:48:14.808 +01:00] [ERR] Error in "ffprobe"
System.Text.Json.JsonException: The JSON value could not be converted to System.String. Path: $.streams[0].start_pts | LineNumber: 32 | BytePositionInLine: 26.
---> System.InvalidOperationException: Cannot get the value of a token type 'Number' as a string.
at System.Text.Json.Utf8JsonReader.GetString()
at System.Text.Json.Serialization.Converters.JsonConverterString.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options)
at System.Text.Json.JsonPropertyInfoNotNullable`4.OnRead(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
at System.Text.Json.JsonPropertyInfo.Read(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
at System.Text.Json.JsonSerializer.HandleValue(JsonTokenType tokenType, JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& state)
at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
--- End of inner exception stack trace ---
at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& readStack, Utf8JsonReader& reader, Exception ex)
at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
at System.Text.Json.JsonSerializer.ReadCore(JsonReaderState& readerState, Boolean isFinalBlock, ReadOnlySpan`1 buffer, JsonSerializerOptions options, ReadStack& readStack)
at System.Text.Json.JsonSerializer.ReadAsync[TValue](Stream utf8Json, Type returnType, JsonSerializerOptions options, CancellationToken cancellationToken)
at MediaBrowser.MediaEncoding.Encoder.MediaEncoder.GetMediaInfoInternal(String inputPath, String primaryPath, MediaProtocol protocol, Boolean extractChapters, String probeSizeArgument, Boolean isAudio, Nullable`1 videoType, Boolean forceEnableLogging, CancellationToken cancellationToken) in /home/pi/dev/jellyfin/MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs:line 399
at MediaBrowser.Providers.MediaInfo.FFProbeVideoInfo.ProbeVideo[T](T item, MetadataRefreshOptions options, CancellationToken cancellationToken) in /home/pi/dev/jellyfin/MediaBrowser.Providers/MediaInfo/FFProbeVideoInfo.cs:line 122
at MediaBrowser.Providers.Manager.MetadataService`2.RunCustomProvider(ICustomMetadataProvider`1 provider, TItemType item, String logName, MetadataRefreshOptions options, RefreshResult refreshResult, CancellationToken cancellationToken) in /home/pi/dev/jellyfin/MediaBrowser.Providers/Manager/MetadataService.cs:line 815
```
* Add support for ffmpeg 4.2
* Parse the complete ffmpeg version instead of only the first 2 digits
* Make max and min version optional
* Remove max limitation (for now)
* Style improvements
1) Reworked FFmpeg and FFprobe path discovery (CLI switch, Custom xml, system $PATH, UI update trigger). Removed FFMpeg folder from Emby.Server.Implementations. All path discovery now in MediaEncoder.
2) Always display FFmpeg path to user in Transcode page.
3) Allow user to remove a Custome FFmpeg path and return to using system $PATH (or --ffmpeg if available).
4) Remove unused code associated with 'prebuilt' FFmpeg.
5) Much improved logging during path discovery.
Bond-009
Claus Vium
boolemancer
nyanmisaka
Stan
BaronGreenback
MrTimscampi
crobibero
Joshua M. Boniface
Fernando Fernández
Max Git
Neil Burrows
Neil Burrows
telans
dkanada
Vasily
artiume
Mark Monteiro
Phallacy
redSpoutnik
PloughPuff
Claus Vium
Vasily
Erwin de Haan