"accu-timer": some changes to repeat_timeout that seem to make it accurate

on Unix and Win32 at speeds up to 500000 timeouts/second (and 700000 on
Linux), and within about .001% as accurate as the system clock.

Fix to Fl_Valuator::step() by Guillermo Andrade.


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.0@1354 ea41ed52-d2ee-0310-a9c1-e6b18d33e121