perspective.grid.preset stored the preset's grid every time, so applying
the preset the document already holds recorded an undo step that changed
nothing. It now compares the new grid with the stored one, as
perspective.grid.set does through grid_kept, and records a step only when
the grid changes. When only whether the grid shows or its active plane
differs, the preset sets them without an undo step, as
perspective.grid.show and perspective.plane.set do.