Fix #109040: Don't calculate homogenous offset if viewspace offset is 0

Users were reporting offset issues when the retopology overlay
was disabled. The reason those issues were happening is because of
`vs_offset = min(vs_offset, vs_z * -0.5);`.
That line is necessary for proper functioning of the retopology
overlay, but causes issues at lower offset values (such as zero, when
the retopology overlay is disabled).

Fixes #109640

Pull Request: https://projects.blender.org/blender/blender/pulls/109657

source/blender/draw/intern/shaders/common_view_lib.glsl

@@ -262,7 +262,11 @@ vec3 point_world_to_view(vec3 p)
  * Offset is in viewspace, so positive values are closer to the camera. */
 float get_homogenous_z_offset(float vs_z, float hs_w, float vs_offset)
 {
-  if (ProjectionMatrix[3][3] == 0.0) {
+  if (vs_offset == 0.0) {
+    /* Don't calculate homogenous offset if viewspace offset is zero. */
+    return 0.0;
+  }
+  else if (ProjectionMatrix[3][3] == 0.0) {
     /* Clamp offset to half of Z to avoid floating point precision errors. */
     vs_offset = min(vs_offset, vs_z * -0.5);
     /* From "Projection Matrix Tricks" by Eric Lengyel: