Edit Mode Selection: Make Vertices not occluded by edges

Debugging the edit mode selection I realized the vertices are often
occluded by edges with the same depth. Sometime it can be the center
pixel of a vertex point and that can lead to some selection issue.

So I increased the offset a bit for the vertices and it seems to fix it.

source/blender/editors/space_view3d/drawobject.c

@@ -319,6 +319,8 @@ void draw_object_backbufsel(
 					bm_wireoffs = bm_solidoffs;
 				}
 
+				ED_view3d_polygon_offset(rv3d, 1.1);
+
 				/* we draw verts if vert select mode. */
 				if (select_mode & SCE_SELECT_VERTEX) {
 					bbs_mesh_verts(geom_verts, bm_wireoffs);