Fix issue 13161 primitiveDisplayString fails to advance destX

Fix issue https://pharo.fogbugz.com/f/cases/13161/BitBltSimulation-primitiveDisplayString-fails-to-advance-destX

Note that destX is a global variable known to the VM.
It must be copied back to BitBlt's inst.var. destX at the end of the primitive.

This fix has been applied both in Squeak VMMaker and Eliot's VMMaker.oscog branch and thus should be safe.