GridGeoSampler: don't change stride of last patch

[adamjstewart]

Partially reverts #630. See #1245 for motivation.

@remtav

[isaaccorley]

Are we sure this works? I thought that to read data including those outside the bounds of an image we have to pass boundless=True like

dest = src.read(
   indexes=band_indexes,
   out_shape=out_shape,
   window=from_bounds(*bounds, src.transform),
   boundless=True
)

[adamjstewart]

Good catch! Without boundless=True, rasterio still gives us an image with the correct shape because we pass in out_shape, but it does so by upsampling the image to match the dimensions. This is why the tests weren't failing. We want it to give nodata pixels instead. Let me fix this.

[adamjstewart]

merge has no boundless parameter, but doesn't need one. I confirmed that it properly samples outside the bounds of the file and uses nodata pixels for padding.

[adamjstewart]

This introduces a serious bug. Changing the CRS of a dataset results in the following error when loading data:

Traceback (most recent call last):
  File "/Users/Adam/torchgeo/test.py", line 11, in <module>
    ds[ds.bounds]
  File "/Users/Adam/torchgeo/torchgeo/datasets/geo.py", line 899, in __getitem__
    samples = [ds[query] for ds in self.datasets]
  File "/Users/Adam/torchgeo/torchgeo/datasets/geo.py", line 899, in <listcomp>
    samples = [ds[query] for ds in self.datasets]
  File "/Users/Adam/torchgeo/torchgeo/datasets/geo.py", line 443, in __getitem__
    data = self._merge_files(filepaths, query, self.band_indexes)
  File "/Users/Adam/torchgeo/torchgeo/datasets/geo.py", line 486, in _merge_files
    dest = src.read(
  File "rasterio/_warp.pyx", line 1233, in rasterio._warp.WarpedVRTReaderBase.read
ValueError: WarpedVRT does not permit boundless reads

The fix is to use rasterio.merge.merge for all I/O instead.