Geometry Nodes: Make plane primitive size consistent

This means the "Size" input is treated as a "diameter" instead of
a "radius", just like the 3D view primitives.

source/blender/nodes/geometry/nodes/node_geo_mesh_primitive_plane.cc

@@ -73,8 +73,8 @@ static Mesh *create_plane_mesh(const int verts_x, const int verts_y, const float
   MutableSpan<MPoly> polys{mesh->mpoly, mesh->totpoly};
 
   {
-    const float dx = size * 2.0f / edges_x;
-    const float dy = size * 2.0f / edges_y;
+    const float dx = size / edges_x;
+    const float dy = size / edges_y;
     float x = -size;
     for (const int x_index : IndexRange(verts_x)) {
       float y = -size;