Overlay: Wireframe: Improve Z-fighting / missing information stippling

To do this we do a half pixel offset in the normal direction.
However, we use the shading vertex normal instead of the geometric normal
(for speed) and this leads to imprecisions.

So we try to mitigate the common case and leave the corner cases as is.

source/blender/draw/engines/overlay/shaders/wireframe_vert.glsl

@@ -102,8 +102,25 @@ void wire_object_color_get(out vec3 rim_col, out vec3 wire_col)
 void main()
 {
   vec3 wpos = point_object_to_world(pos);
+  vec3 wnor = normalize(normal_object_to_world(nor));
+
+  bool is_persp = (ProjectionMatrix[3][3] == 0.0);
+  vec3 V = (is_persp) ? normalize(ViewMatrixInverse[3].xyz - wpos) : ViewMatrix[2].xyz;
+
+  float facing = dot(wnor, V);
+  float facing_ratio = clamp(1.0 - facing * facing, 0.0, 1.0);
+  float flip = sign(facing);           /* Flip when not facing the normal (i.e.: backfacing). */
+  float curvature = (1.0 - wd * 0.75); /* Avoid making things worse for curvy areas. */
+  vec3 wofs = wnor * (facing_ratio * curvature * flip);
+  wofs = normal_world_to_view(wofs);
+
   gl_Position = point_world_to_ndc(wpos);
 
+  /* Push vertex half a pixel (maximum) in normal direction. */
+  gl_Position.xy += wofs.xy * sizeViewportInv.xy * gl_Position.w;
+
+  /* Push the vertex towards the camera. Helps a bit. */
+  gl_Position.z -= facing_ratio * curvature * 1e-4;
 
   /* Convert to screen position [0..sizeVp]. */
   edgeStart = ((gl_Position.xy / gl_Position.w) * 0.5 + 0.5) * sizeViewport.xy;
@@ -119,8 +136,6 @@ void main()
     wire_color_get(rim_col, wire_col);
   }
 
-  vec3 wnor = normalize(normal_object_to_world(nor));
-  float facing = dot(wnor, ViewMatrixInverse[2].xyz);
   facing = clamp(abs(facing), 0.0, 1.0);
 
   vec3 final_front_col = mix(rim_col, wire_col, 0.4);