Fix #130368: GPv3: Canvas overlay incorrect offset

The issue was that if the evaluated object doesn't have an active
layer, the canvas is offset by twice the objects position.

The reason was that the code scaled the whole transformation
matrix by two to match the code in 4.2, but then had to overwrite
the location of the transformation again to counteract the scaling.

The fix is to not counteract the scaling and just scale the 3x3 part
of the matrix instead. This way we can remove the part afterwards
that writes to the location of the transform.

Pull Request: https://projects.blender.org/blender/blender/pulls/130454

source/blender/draw/engines/overlay/overlay_grease_pencil.cc

@@ -244,16 +244,13 @@ void OVERLAY_grease_pencil_cache_init(OVERLAY_Data *vedata)
       break;
   }
 
-  /* Note: This is here to match the legacy size. */
-  mat *= 2.0f;
-
   if (ts->gpencil_v3d_align & GP_PROJECT_CURSOR) {
     mat.location() = cursor->location;
   }
-  else if (grease_pencil.has_active_layer()) {
-    const bke::greasepencil::Layer &layer = *grease_pencil.get_active_layer();
-    mat.location() = layer.to_world_space(*ob).location();
-  }
+
+  /* Note: This is here to match the legacy size. */
+  mat.view<3, 3>() *= 2.0f;
+
   /* Local transform of the grid from the overlay settings. */
   const float3 offset = float3(
       v3d->overlay.gpencil_grid_offset[0], v3d->overlay.gpencil_grid_offset[1], 0.0f);