Mesh Gradient: Rows and Columns in the tool options should change existing mesh gradients

Summary:

Currently the Rows and Columns options in the mesh gradient tool only applies to newly-created mesh gradients. It would be nice if it applies to existing mesh gradients (though a more proper row/column deleting methoud would be better).

Steps to reproduce:

• open Inkscape
• draw a rectangle
• add a mesh gradient onto the rectangle
• modify the value of "Rows" or "Columns" in the tool options

What happened?

Nothing.

What should have happened?

The rows and columns are added or deleted.

Version info

Inkscape 1.3.2 (091e20ef0f, 2023-11-25)

    GLib version:     2.80.0
    GTK version:      3.24.41
    glibmm version:   2.66.7
    gtkmm version:    3.24.9
    libxml2 version:  2.12.6
    libxslt version:  1.1.39
    Cairo version:    1.18.0
    Pango version:    1.52.2
    HarfBuzz version: 8.4.0

    OS version:       Slackware 15.0 x86_64 (post 15.0 -current)