Fix portability concerns in halide_image_info.h
Fix portability concerns in halide_image_info.h

— variable-length arrays are not supported by all compilers; use
std::vector instead
— The “Buffer<>” type used must be a Halide::Runtime::Buffer, but the
Runtime namespace isn’t necessarily opened; add explicitly Runtime:: to
clarify this.

Former-commit-id: ed805d293f996417d56c22e9ab4c4ba299988f58
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide