glx: Add implementation of __GLXContext->loseCurrent for direct ctxts

This adds a dummy implementation for the loseCurrent function in
__GLXContext for direct contexts which just returns GL_TRUE. Without
this then the X server can crash if receives a MakeCurrent message for
a direct context because it will attempt to call loseCurrent when
cleaning up the client in the callback for ClientStateGone.

[ajax: added assumed s-o-b line]

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86531
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Neil Roberts <neil@linux.intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>