RPC transform: add RPC_DEM_MISSING_VALUE transformer option to avoid failure when there's no DEM at the transformed point (#5730)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27930 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_mingw_w64