Sketcher: preserve local overlay depth for dimensions and grid

[tritao]

This fixes depth layering for face-attached sketches in edit mode.

The branch does two things:

1. Preserve the local Sketcher overlay Z for dimension/helper lines.
2. Lift the Sketcher grid slightly off the support face so it remains visible.

Problem

When editing a sketch attached to a face, dimension/helper lines could disappear behind the model geometry.

The underlying issue was that some SoDatumLabel drawing paths used glVertex2f(...), which discards the incoming Z coordinate. That flattened those lines onto the sketch plane instead of using the local Sketcher overlay depth that had already been assigned upstream.

With depth testing enabled, those lines could then lose against the support face.

After correcting that, the grid exposed the same problem from the opposite direction: it was still generated directly on the support plane, so it rendered behind the face while dimensions were correctly using their local overlay depth.

Fix

Preserve local Z for dimension lines

In src/Gui/SoDatumLabel.cpp, the affected dimension/helper line paths now use the full 3D input points instead of dropping Z.

This makes those lines render at the same local Sketcher overlay depth they already receive from the constraint code, including the correct front/back behavior.

Lift the grid off the support face

In src/Mod/Part/Gui/ViewProviderGridExtension.cpp, the grid is now emitted with a small local Z offset toward the active viewing side instead of being placed directly on the support plane.

This keeps the grid visible while still leaving it below the rest of the sketch overlay.

Result

• dimension lines no longer disappear behind the support geometry
• the Sketcher grid remains visible
• both now follow the existing local Sketcher depth layering instead of using conflicting plane-level depth

Fixes #27924

[maxwxyz]

Tested

[freecad-ci-runner]

Successfully created backport PR for releases/FreeCAD-1-1:

• [Backport releases/FreeCAD-1-1] Sketcher: preserve local overlay depth for dimensions and grid #29289