Fix incorrect GLX pbuffer attributes terminator

The glXCreatePbuffer() documentation states that the attributes list
must be terminated with "None or NULL". However, GLX_NONE does not
have a null value. Mesa is sensitive to this.

Change-Id: I9606d95a6a0dadec446496abb8e0213950e3d700
Reviewed-by: Michael Brüning <michael.bruning@qt.io>