title: Wind barbs and vectors in Cartopy - issue with regrid_shape
body: <p>I've recently switched from Basemap to Cartopy and am having some trouble plotting wind vectors. I've got a North Polar Stereographic projection (central_longitude=-100.0) with extent [-180,180,10,90]. I have a netCDF dataset with u,v wind values every 0.5 deg latitude and longitude. </p>
  
  <p>If I plot the barbs using slice to manually skip a certain number (since the data is too dense to plot all of it), I get what appears to be a correct representation of the field (see first image below):</p>
  
  <pre><code>sknum = 15
  skip=(slice(None,None,sknum),slice(None,None,sknum))
  ax.barbs(lons[skip], lats[skip], u[skip],v[skip], length=6,
               sizes=dict(emptybarb=0.25, spacing=.2, height=0.5),
               zorder = 20,
               linewidth=0.95, transform= ccrs.PlateCarree())
  </code></pre>
  
  <p>However, this method looks a bit unnatural and is too dense near the pole. Using the regrid_shape feature of cartopy gives me a worse error (see second image), with rows of the same vector over and over.</p>
  
  <pre><code>ax.barbs(lons, lats, u,v, length=6,
           sizes=dict(emptybarb=0.25, spacing=.2, height=0.5),
           zorder = 20,
           linewidth=0.95, transform= ccrs.PlateCarree(),  regrid_shape=20)
  </code></pre>
  
  <p>Is this a bug for regrid_shape under this projection, am I doing something wrong, and/or is there another way for me to get natural-looking vectors? </p>
  
  <p><a href="https://i.stack.imgur.com/gQTUn.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/gQTUn.jpg" alt="Manual skip image"></a>
  <a href="https://i.stack.imgur.com/YzHl0.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YzHl0.jpg" alt="Regrid_Shape = 20 image"></a></p>