Re: Use of GCPS in Rasterio

vincent.sarago@...
 

Sean, my main worry is that we need to use `WarpedVRT` so it adds one more layer, and thus not optimal. But it will be better than nothing :-) 

For files with gcps, not only the transform but also the CRS won't be set by default so basically, the WarpedVRT reader it should do something like: 

```
gcps, gcps_crs = src.gcps
in_transform = from_gcps(gcps) if gcps else src.transform
in_crs = gcps_crs if gcps else src.crs

# in https://github.com/mapbox/rasterio/blob/master/rasterio/_warp.pyx#L738
self.src_transform = src_transform if src_transform else in_transform

# in https://github.com/mapbox/rasterio/blob/master/rasterio/_warp.pyx#L736
self.src_crs = src_crs if src_crs else in_crs
```

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