VRT: implement heuristics to determine if GetMinimum()/GetMaximum() should use the implementation of their sources of not. Can be overriden by setting VRT_MIN_MAX_FROM_SOURCES = YES/NO (#5444)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27540 f0d54148-0727-0410-94bb-9a71ac55c965