This allows MemoryReadWriteStream to be passed successfully to
functions that use the SeekableReadStream type so that they can
call the `pos` method, like the DPCMStream class of the VMD
decoder.
MemoryReadWriteStream now returns int32, not uint32. It actually doesn't
ever return -1 to indicate that an error occured, so uint32 was a better
choice, but that's what is used in WriteStream base class now.
That method is abstract, so that's also why OutSaveFile had to override
it.
Now it is based on MemoryReadWriteStream, which is introduced by this
commit. This stream is using ring buffer and is dynamically increasing
its size when necessary.
It returns int32, because some complex streams are derived from both
WriteStream and ReadStream, and in ReadStream pos() returns int32,
because -1 means an error.
I had to change MemoryStream's uint32 to int32, which is probably not so
good.
