Blender Git Loki

GP: Define new Draw Mode

Add a new Draw Mode to display panel in order to define the z-.depth order of the strokes using the real 3D position and not the 2D layer position.

This change makes possible to use VR with grease pencil drawings because the depth of the strokes change with camera position. Also, provide an alternative solution to tasks: T57859, T60325,

The parameter only works with 3D space depth ordering. The Back and Front depths are incompatible with 3D Space mode.

Options are:
- Back
- Front
- 3D Space->2D Layers (default)
-3D Space->3D Location (new mode)