WinGui: Improve coverart infrastructure code.

libhb/handbrake/handbrake.h

libhb/hb.c

win/CS/HandBrake.Interop/Interop/HandBrakeInstance.cs

win/CS/HandBrake.Interop/Interop/HbLib/HbFunctions.cs

win/CS/HandBrake.Interop/Interop/HbLib/hb_coverart_s.cs

win/CS/HandBrake.Interop/Interop/Interfaces/IScanInstance.cs

...terop/Interop/Interfaces/Model/Preview/RawCoverartData.cs

win/CS/HandBrakeWPF/HandBrakeWPF.csproj

win/CS/HandBrakeWPF/Instance/RemoteScanInstance.cs

win/CS/HandBrakeWPF/Model/CoverArtImage.cs

win/CS/HandBrakeWPF/Services/Scan/Interfaces/IScan.cs

win/CS/HandBrakeWPF/Services/Scan/LibScan.cs

win/CS/HandBrakeWPF/Utilities/BitmapUtilities.cs

win/CS/HandBrakeWPF/ViewModels/MetaDataViewModel.cs

win/CS/HandBrakeWPF/Views/Images/EmptyCoverArt.png

win/CS/HandBrakeWPF/Views/MetaDataView.xaml