More ref/const-ref conversions I missed; add missing copy_to_host() call in save_ppm().
More ref/const-ref conversions I missed; add missing copy_to_host() call in save_ppm().


Former-commit-id: e378bff907d5864f7567826cd71cd62920f448f3
Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide