Negatively scaled objects should be mirrored. This is already implemented, but it breaks when mirrored and non-mirrored instances of the same object are visible together.
It turns out that the code that skips-over repeats in `RenderForwardClustered::_render_list_template` also skips the code that accounts for the culling mode of mirrored objects.
The solution here is to consider the `mirror` flag when determining repeats. This might result in more draw commands than necessary since a mirrored object can split a group of non-mirrored instances in two.
This problem doesn't appear in the mobile renderer because the repeat optimization isn't implemented there yet.
The problem still appears in MultiMeshInstance3D in *all* renderers.
Fixes#62879 and #58546.
79d3e090cd