Remove redundant vertices from HeightMapShape3D debug mesh

[kleonc]

• HeightMapShape3D::get_debug_arraymesh_faces was creating 4 vertices (position+color) per grid quad instead of a single vertex per grid point. Made it not generate duplicated vertices. The mesh layout/topology remains unchanged (hence index count is the same before/after).

map_width⨯map_depth	(Before) vertex count	(After) vertex count
W⨯D	(W - 1) * (D - 1) * 4	W * D
2⨯2	4	4
10⨯10	324	100
100⨯100	39 204	10 000
1000⨯1000	3 992 004	1 000 000

• Fixed HeightMapShape3D::get_debug_arraymesh_faces erroring when map_width == 1 or map_depth == 1 (which are allowed by relevant setters).

• Made HeightMapShape3D::get_debug_mesh_lines() not use .write() while generating line points.