Its members can be on copper layers, but the group itself isn't on any
layer.
Also fixes a bug where we were trying to clone TRIANGULATED_POLYGON::TRI
shapes as indexable sub-shapes. (The TRI only has indexes into its
parent, so cloning it will only result in segfaults down the line.)
Also fixes a bug where we weren't including copper items inside groups
when checking footprint net ties.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/15021
(cherry picked from commit 40abb013ec)
b857374aa2