title: Using setup_params() in a ggplot2 geom
body: <p>I'm working on making my own geom for ggplot2, and I've noticed that in ggplot2's documentation, it explicitly says that there is no <code>setup_params()</code> for geoms:</p>
  <blockquote>
  <p>Compared to <code>Stat</code> and <code>Position</code>, <code>Geom</code> is a little different because the execution of the setup and compute functions is split up. <code>setup_data</code> runs before position adjustments, and <code>draw_layer()</code> is not run until render time, much later. This means there is no <code>setup_params</code> because it's hard to communicate the changes.</p>
  </blockquote>
  <p>However, when I make a ggproto Geom that has <code>setup_params = function(data, params) {...}</code>, it calls that function fine, and it seemingly works.</p>
  <p>What gives? Can I use this function to modify parameters or not?</p>