Fix Cairo drawing in test/cairo_test demo for Windows (#694)

On the Windows platform Cairo drawings must be flushed explicitly.
This is done in Fl_Cairo_Window after calling the draw callback but
it was neither done nor documented in test/cairo_test.cxx when using
another type of Window with its overloaded draw() method, i.e. when
FLTK was configured with --enable-cairoext or CMake OPTION_CAIROEXT.

Note: user code must either explicitly flush the Cairo drawings as
done here or destroy the Cairo context which is not done in this demo
because the Cairo context should be kept alive.

0 files changed, 0 insertions, 0 deletions