We instead use a handle reference counter on the GPUVertBufs used by
the instancing batches. This make sure that if an update happens on the
GPUVertBuf used to contruct the batch, they will never have the same
memory address than the previously allocated ones (since they are still
pending deletion thanks to the refcounter).
This avoid the linear search to update the GPUBatch in the case a
batch is deleted (which was even a bad option since they could be only
cleared)
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