GEOT-4836, fixing resolution computation in mosaic

Conflicts:
	modules/plugin/imagemosaic/src/main/java/org/geotools/gce/imagemosaic/RasterLayerRequest.java
	modules/plugin/imagemosaic/src/main/java/org/geotools/gce/imagemosaic/SpatialRequestHelper.java
	modules/plugin/imagemosaic/src/test/java/org/geotools/gce/imagemosaic/ImageMosaicReaderTest.java
Merge pull request #574 from n-lagomarsini/GEOT-4836_10x

GEOT-4836, fixing resolution computation in mosaic (Backport on 10.x)