Used to apply by_group overrides to fills, patterns, edges, labels,
and quantities for a single panel produced by euler(..., by = ...). The
override list contains gpar-level fields (and optionally rot), which are
overlaid onto param$gp (and param$set_gp for shape-mode patterns).
Structural fields are not overlaid here — they must be filtered upstream.
apply_panel_overrides(param, override)The param list with overlaid fields.
a list with $gp (a grid::gpar) and optionally $rot and/or
$set_gp.
a flat named list of fields to overlay.