Fix return value of wxCountingOutputStream::LastWrite().

Fix return value of wxCountingOutputStream::LastWrite().

Don't reuse m_lastcount in wxCountingOutputStream to store the stream length,
this doesn't make any sense and results in LastWrite() returning completely
wrong results as it expects m_lastcount to be the number of bytes written by
the last operation.

Add m_lastPos member to store the stream length instead.

Also correct wxCountingOutputStream documentation.

Closes #15215.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74036 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775