Optimize xml_buffered_writer::write(char_t...)
Optimize xml_buffered_writer::write(char_t...)

Make it easier for the compiler to generate good code by loading bufsize
into a local once and returning new offset from flush(). This results in
7% performance gain.

git-svn-id: 3715ede575e2934be0fecc9022d600f6f7ea101b@1024 99668b35-9821-0410-8761-19e4c4f06640

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml