Blender Git Loki

Fix T40567 Crash when returing to object mode from dyntopo.

Returning to object mode reactivates any generative modifiers
and this can lead to a polycount explosion.

For now just improve the warning when entering dyntopo with generative
modifiers. I would like to add the ability spawn options to apply or
remove the modifiers too, however separate undo stack system comes back
with a vengeance here, since it won't allow restoring the application/
also may invalidate all sculpt undo in the undo stack prior to the
application (needs investigation).