fpspreadsheet: Use TBufStream as general-purpose stream if woBufStream is set in Worksheet.WritingOptions. woBufStream replaces woSaveMemory. Results in a significant speed enhancement for biff2 (64000x100 cells -> 31 sec without, 1.7 sec with woBufStream).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3337 8e941d3f-bd1b-0410-a28a-d453659cc2b4