BUG: #2046 Add an epsilon margin to compute the m_ReachableOutputRegion

The application Superimpose crashed with a segmentation fault during
resampling. Investigation revealed that the interpolated index was out
of image bounds. The application uses the PHR mode and resample the XS
image with a otb::GridResampleImageFilter. This filter doesn't check
explicitly that input indexes are in the input buffered region. It
uses a m_ReachableOutputRegion to crop the output region
processed. The problem appears because an output pixel is right on the
border of the input image extent, and when someone uses a nearest neighbor
interpolator, this one converts the continuous index to an
out-of-bound index.