Blender Git Loki

Fix T43867: Clicking outside the viewport exits now Grease Pencil "Continous Draw" mode

When working with a pen only, it was previously impossible to exit Grease Pencil
draw mode (when continous drawing was enabled). Now, clicking outside the drawing region
(e.g. in the timeline, properties editor, or the header/properties/toolshelf regions)
where you are drawing will exit this mode.

Some corner cases to be aware of:
* When Region Overlap is enabled, clicking on the overlapping panels still exits
draw mode (even though you can see behind the buttons)
* In the Nodes Editor, clicking on a node (while in drawmode) will still draw a
dot/stroke. But, you can still exit drawmode by clicking on any of the panels
(properties/toolshelf/header) mentioned earlier
* To cope with cases where the operator is launched from the toolshelf, the
code now sets a new "active region" when the first stroke is performed
(based on what region is under the cursor at the time of that stroke),
overwriting the setting that got stored when invoking the operator (i.e. the toolshelf).
This change doesn't have any real user-visible effects, other than making it possible for
this fix to actually work without breaking that use case.