server: Add a process flag indicating whether the process is terminating, use it to block thread creation in a being terminated process.

winemac: Mouse drags don't imply anything about z-order of target window because of implicit capture.

This fixes an issue with sub-pixel-precise pointing devices on Lion and later.
Wine's notion of the cursor position would get out of sync with the actual position
because deltas don't convey the actual movement distance.