#12 [X11] Cursor-creation failure

[Richard Rauch]

In the X11 branch of the codee, in glutSetCursor(), if
we fail to create a cursor, the cursor ID is set to
{None}. If we blithely pass that into the X function
for setting a cursor, then this unsets the custom cursor
for the window (i.e., acts as if the client had
requested us to set GLUT_CURSOR_INHERIT).

This won't crash, but is not really all that desirable.

The code is documented as leaving this behavior
undefined so that we have room to change our minds. The
most desirable thing is probably to discard the request
and somehow indicate the failure.