Caused by rBc476c36e4008.
This hooks into the existing FIXME (workaround for a missing update
tagging), reactivates the NS_VIEW3D_GPU notifier (introduced in
rB2ad3d8f158d2 -- but not going anywhere atm.) to check changes to rv3d-
rflag which indicated UserStudioLight has changed. To not have updates
all the time, the rflag also needs to be cleared again (see original
rB2ad3d8f158d2), this is now done in 'workbench_draw_finish'.