Blender Git Loki

Fix inaccuracy of SCREEN_OT_area_split when snap to midpoint and adjacent.

By default when moving a edge of the screen it always snaps to an invisible grid with unit of 4 pixels.
This was also affecting the snap to the midpoint and adjacent.
The solution was to make the snap to areagrid optional and use values of `origmin` and `origsize` that match the transformations in screen_edit.c.