Commit 9e2baefb38ba30b5603bb5280e55d45753531096 ensures we use a consistent
VM mode (global/local) through the sampling of the halftone function sampling.

But the mode stored on the exec stack, to reset when sampling was complete,
was the VM mode used for sampling, rather than the current mode when sampling
started.

Caused an invalidaccess error with:

-sDEVICE=pkmraw -r72 -o stuff%03d.pkm ../../tests_private/comparefiles/Bug690683.ps