title: png() function in R fail to create png file with dimension larger than 3000x3000px
body: <p>I have trying to generate some map using <code>ggplot</code> and <code>sp</code>, I have a basemap with the original size of <code>3000+ x 3000+</code> pixels, I also have some with <code>2000+ x 2000+</code> pixels. Interestingly, until now I still cannot create the <code>3000+ x 3000+</code> pixel png files, as R gives me the following error:</p>
  
  <pre><code>Error in png(chart.filename, width = basemap.xlength, height = basemap.ylength,  : 
    unable to start png() device
  In addition: Warning messages:
  1: In png(chart.filename, width = basemap.xlength, height = basemap.ylength,  :
    Unable to allocate bitmap
  2: In png(chart.filename, width = basemap.xlength, height = basemap.ylength,  :
    opening device failed
  </code></pre>
  
  <p>Is that a limitation to R? Can how I get through it? Thanks.</p>
  
  <p>I am using Win7 with R 2.15.0.</p>