Fix some IStreamStack and SharpCompressStream functions

[Morilli]

• resolves ExtractAllEntries returns no entries when reading a ZIP from a Stream #960

Buffer handling seems to have been completely broken.
I've fixed the handling in IStreamStack.StackSeek that basically duplicated the logic from SharpCompressStream.Seek (while not actually working correctly), fixed incorrect behavior of the IStreamStack.BufferPosition setter and fixed and off-by-one in SharpCompressStream.Seek.

I suspect there are more issues in StackSeek that may or may not even be relevant for how buffered IStreamStacks are even used. It's hard to tell because the method documentation does not seem to fully match the implementation.

This allows reverting #964 now that the underlying issue has been fixed.

TODO:

• make some tests for this to validate behavior

[adamhathcock]

Thanks, I'll take a look at this. Seek and setting BufferPosition should be the same thing?

[Morilli]

Seek and setting BufferPosition should be the same thing?

For a buffered stream, it is the same if the seek would only seek to somewhere inside of the currently buffered region. Unfortunately some buffered streams aren't seekable so we can't unconditionally call seek without trying to set the buffer position beforehand. This seems to be mainly required for archive detection logic as that uses a buffered, non-seekable stream for rewind logic where we need to be able to rewind the buffer position while not having access to the Seek method.