fix for glitch in X11 with tablets.

Notices this while using continuous-grab, since this is disabled when the tablet is being used.
Quite often I would use the tablet then drag a button with the mouse but blender would still have the tablet enabled.
This error would cause other parts of blender to behave incorrectly too since wmEvents would have wmTabletData set, operators check for this in some cases.


The problem was blender didn't reliably get ProximityOut events, eg:
  moving the cursor outside the window with the tablet, then back over the window with the mouse -
  meant blender didn't get a 'ProximityOut' event and would keep the active stylus value set.

For now, when the processing events and the active stylus is set, run a check that the tablet is still in proximity.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53850 954f8c5b-7b00-dc11-b283-0030488c597c