VRT: implement non-linear scaling with a power function (addition of Exponent, SrcMin, SrcMax, DstMin, DstMax sub-elements in <ComplexSource>); add a -exponent option to gdal_translate to be used with -scale

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26092 f0d54148-0727-0410-94bb-9a71ac55c965