Some Win32 drivers would draw into wrong buffers

after OpenGL mode change.
After changing from single- to doublebuffered OpenGL, glClear would
write into the FRONbuffer and swap_buffers would not send a glFlush().
(nVidia GeForce2 MX/MX 400, 2560x1024 pixel, 16bit)


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.1@1805 ea41ed52-d2ee-0310-a9c1-e6b18d33e121