title: rasterio what to do when nodata value is present in data
body: <p>I have a geotiff raster (uint8 datatype) that I'm reading in with rasterio and then cropping with a polygon.</p>
  
  <p>But the issue is that all pixels outside the polygon should be considered nodata in the final cropped image. But, I'm not sure what nodata value to use because my raster contains most values. So if I set the nodata value to be 0 for example, there would be 0 values within the raster itself that would be marked as nodata, even though they shouldn't be.</p>
  
  <p>So is there some way to set the mask without caring what the actual data value at that pixel is? Ideally I could just set mask values to some special number that I know no data value would have.</p>
  
  <p>Or is the only way to convert my uint8 raster to a float32, and set the nodata to be <code>nan</code>? Would there be any side effects from doing this data type conversion on a raster?  </p>