Fix for #35495, Material preview do not update. This was caused by a line inserted by pynodes merge that would cancel all running preview jobs on each node area refresh. Not sure what the rationale for
this behavior was, but it certainly doesn't work correctly.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57050 954f8c5b-7b00-dc11-b283-0030488c597c