[GEOS-6328]: use direct read when creating the small sample image

[GEOS-6328]: use direct read when creating the small sample image

I have tested it against a 60GB mosaic as well as with a 100 granules mosaic.
No issues when creating the coverage, since coverageBuilder read a 5x5 pixels from a single granule in any case.

The code change also includes some minimal reformatting due to bad tabs or spaces there.