Reflect size aesthetic in key_glyph_phylopic

[willgearty]

The sizes that we specify in geom_phylopic are in y-axis units, but we don't have access to the y-axis object when plotting the legend keys, so we have no idea how to scale the keys properly to match the sizes in the plot (relative sizes will always be correct, but the absolute sizes will be off). I'm not exactly sure how to get around this (we plot the legend key before we draw the panel, so we can't just save the y-axis range in phylopic_env), but I'm making this issue to record that this would be nice to have.