Replace use of SharpZipLib with standard .NET DeflateStream

SharpZipLib is used in only one place, to inflate zTXt chunks in PNG files.
DeflateStream can do the same job, without introducing a dependency
or complicating nuget packaging.
Since DeflateStream is a built-in .NET class, there is no longer a reason to
make zTXt support conditional, so I removed the conditional compilation.
Also removed a small defect in the VS project file which prevented compilation using VS.