Add RAII struct for PNG row buffer allocation
Add RAII struct for PNG row buffer allocation

Also, drive-by constification of FileOpener


Former-commit-id: da60c1bbbceb1cf15cd1681ee3b10eed44082c48
darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide