title: The draw_image() function from cowplot results in blurred pdfs
body: <p>Reading a vectorized pdf sometimes result in a badly blurred image with the <code>draw_image()</code> function from cowplot:</p>
  
  <pre><code>library(ggplot2)
  library(cowplot)
  library(magick)
  
  # make pdf input as example
  p &lt;- ggplot(iris, aes(Sepal.Length, Sepal.Width, shape = Species)) + 
    geom_point() + scale_shape_manual(values = 21:23) + theme_classic()
  ggsave("input.pdf", p, width = 6, height = 4.2)
  
  # now draw with draw_image() and then write as png
  fig &lt;- ggdraw() + draw_image("input.pdf")
  ggsave("output.png", fig, width = 1, height = .7, dpi = 1200) # blurred image
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/6M0ua.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6M0ua.png" alt="enter image description here"></a></p>
  
  <p>However, reading SVGs works fine:</p>
  
  <pre><code>fig &lt;- ggdraw() +
    draw_image("http://jeroen.github.io/images/tiger.svg")
  ggsave("output.png", fig, width = 1, height = .7, dpi = 1200)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/vXy1P.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vXy1P.png" alt="enter image description here"></a></p>
  
  <p>Also, using:</p>
  
  <pre><code>magick::image_read_pdf("input.pdf")
  </code></pre>
  
  <p>results in a non-blurry output.</p>