Mask primitives

Currently only circle and square, might be easily
extended in the future.

New primitives are creating at cursor location.
This also implied adding 2d cursor to space clip.

Also fix set 2D cursor location which didn't work
in image editor's mask mode since 2.67.

TODO: draw_image_cursor better be moved to some
      more generic file, but it's not so much
      important for now and might be solved later.

Thanks Campbell for the review!

release/scripts/startup/bl_operators/__init__.py

@@ -28,6 +28,7 @@ _modules = [
     "clip",
     "console",
     "image",
+    "mask",
     "mesh",
     "node",
     "object_align",