Fixed AudFormat and WavFormat implementations of ISoundFormat.LengthInSeconds

- Fixed a rounding issue in `WavReader.WaveLength()`.
- Fixed `AudReader.SoundLength()` not resetting the stream position.
- Fixed crashes caused by disposed streams because `LengthInSeconds` would try and calculate the length on the fly. It is now precalculated and cached (making it consistent across all 5 current `ISoundFormat` implementations).
- Fixed a crash in `AudReader.LoadSound()`'s `out Func<Stream> result` because that func would try and access the disposed stream's `Length` property. That works for `SegmentStream`, but not for `FileStream`.
- Fixed frameCount/soundLength label positioning in the AssetBrowser window to avoid text clipping .

OpenRA.Mods.Common/AudioLoaders/WavLoader.cs

@@ -53,7 +53,7 @@ namespace OpenRA.Mods.Common.AudioLoaders
 		public int Channels => channels;
 		public int SampleBits => sampleBits;
 		public int SampleRate => sampleRate;
-		public float LengthInSeconds => WavReader.WaveLength(sourceStream);
+		public float LengthInSeconds { get; }
 		public Stream GetPCMInputStream() { return wavStreamFactory(); }
 		public void Dispose() { sourceStream.Dispose(); }
 
@@ -69,6 +69,8 @@ namespace OpenRA.Mods.Common.AudioLoaders
 
 			if (!WavReader.LoadSound(stream, out wavStreamFactory, out channels, out sampleBits, out sampleRate))
 				throw new InvalidDataException();
+
+			LengthInSeconds = WavReader.WaveLength(sourceStream);
 		}
 	}
 }