warp: parallelize using gegl-parallel
gegl:warp can't use auto-threading. Use gegl-parallel to parallelize it manually. Convert the op to C++ in order to use lamdas, which simplify parallelizarion. Move the op to a new common-cxx/ module, where we can add C++ operations, and which is only built when a suitable C++ compiler is found. We don't leave it in common/, since Automake seems to always use CXXLD to link libraries that have (potential) C++ sources, even if they're if-ed out after configuration.
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