Fl_Gl_Window does not set drawbuffer(BACKBUFFER) for single-buffered

windows.

Fl_Input::replace(...) correctly updates the display if the replaced
region does not include the mark, point, or selected region.

Added Fl::add_check(...), Fl::remove_check, and Fl::has_check. These
are similar to idle callbacks but are only called just before it waits
for new events. They can be used to watch for changes in global state
and respond to them.


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.0@1347 ea41ed52-d2ee-0310-a9c1-e6b18d33e121