The GridMap editor's grid previously did not not move when moving
the tile cursor, only on certain menu actions.
This commit adds an update_grid call for valid mouse movement in
the editor, changes update_grid to only offset if there's a valid
cursor, and adds updates for the grid and cursor when a mesh library
is changed since we need the new RID for the update_grid check,
and if we remove the mesh library we want to make sure it gets
recentered. As a nice side-effect, it also updates to the new cursor
mesh if you load a new mesh library.
Fixes#57068
0defaefdba