MRF: fix 'narrowing conversion from ‘GInt32’ to ‘size_t
MRF: fix 'narrowing conversion from ‘GInt32’ to ‘size_t

MRF: fix 'narrowing conversion from ‘GInt32 {aka int}’ to ‘size_t {aka
long unsigned int}’ inside { } [-Wnarrowing]' with gcc 4.8 -std=C++11
Found during GDAL testing:

https://github.com/OSGeo/gdal/commit/d141531a22c7f4f5318c48760992305e77666614

nasa-gibs/mrf,nasa-gibs/mrf,nasa-gibs/mrf