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/FileFormats/WavReader.cs

@@ -123,7 +123,7 @@ namespace OpenRA.Mods.Common.FileFormats
 			var bitsPerSample = s.ReadInt16();
 			var length = s.Length * 8;
 
-			return length / (channels * sampleRate * bitsPerSample);
+			return (float)length / (channels * sampleRate * bitsPerSample);
 		}
 
 		sealed class WavStreamImaAdpcm : ReadOnlyAdapterStream