Based on the previous overlay shader from merwin.
This shader takes care of clipped vertex cases and do all edit mode face info in one pass (except face centers).
As the shading is done one the triangle itself the visual can't go beyond the surface of the mesh. That leads to half displayed edges on the outline of the mesh.
This problem can be fixed by a second pass.
This is work in progress.
This should give the overall direction to whom wants to finish it.
- Renamed EDIT mode engine to EDIT_MESH mode engine
- Introduce EDIT_ARMATURE mode engine
- Started to port legacy drawarmature.c to draw_armature.c
