title: error when 3d graphics with knitr and rgl
body: <p>I am trying to include an interactive plot in my htlm output using knitr and rgl. I have had success in the past but I am now unable to produce graphics from standard examples. When I run the code below my output includes the error "Your browser does not support WebGL. See <a href="http://get.webgl.org" rel="nofollow">http://get.webgl.org</a>". When I check if it is supported my browser works fine with others interactive graphics. </p>
  
  <p>any help would be appreciated, I may be making a simple mistake.  </p>
  
  <pre><code>---
  output:
    html_document:
      keep_md: yes
  ---
  ```
  ```{r setup, results='asis'}
  library(rgl)
  library(knitr)
  knit_hooks$set(webgl = hook_webgl)
  ```
  
  ```{r testgl, webgl=TRUE}
  x &lt;- sort(rnorm(1000))
  y &lt;- rnorm(1000)
  z &lt;- rnorm(1000) + atan2(x,y)
  plot3d(x, y, z, col=rainbow(1000))
  ```
  </code></pre>