... "when there's no current window".
Silently ignore GLUT function calls that need a current window if the
current window is NULL, return 0 from functions that return an 'int'.
Check if window is shown in Fl_X11_Gl_Window_Driver::swap_buffers().
This would issue "XRequest.nnn: GLXBadDrawable 0x0" on X11 otherwise.
Note: the chosen implementation to ignore GLUT calls silently appears
to be compatible with GLUT (3.7) whereas FreeGLUT 3.0 would issue error
messages and exit. The latter could be implemented as well but would
be much more work.
