Optimize grad_to_dp from 4s to 1s

[sirynka]

> ./run.sh
++ cd ./seam-carving-tsoding
++ ./nob ./images/Broadway_tower_edit.jpg ./images/Broadway_tower_squashed.jpg
[INFO] directory `./build/` already exists
[INFO] ./build/stb_image.o is up to date
[INFO] ./build/stb_image_write.o is up to date
[INFO] CMD: cc -Wall -Wextra -ggdb -O3 -o ./build/main main.c ./build/stb_image.o ./build/stb_image_write.o -lm
[INFO] CMD: ./build/main ./images/Broadway_tower_edit.jpg ./images/Broadway_tower_squashed.jpg
OK: generated ./images/Broadway_tower_squashed.jpg
[INFO] Resizing took 4.092352secs
++ echo

++ cd ./seam-carving-dp
++ ./nob ./images/Broadway_tower_edit.jpg ./images/Broadway_tower_squashed.jpg
[INFO] directory `./build/` already exists
[INFO] ./build/stb_image.o is up to date
[INFO] ./build/stb_image_write.o is up to date
[INFO] CMD: cc -Wall -Wextra -ggdb -O3 -o ./build/main main.c ./build/stb_image.o ./build/stb_image_write.o -lm
[INFO] CMD: ./build/main ./images/Broadway_tower_edit.jpg ./images/Broadway_tower_squashed.jpg
OK: generated ./images/Broadway_tower_squashed.jpg
[INFO] Resizing took 1.071051secs
++ echo

++ cmp -s \
    ./seam-carving-tsoding/images/Broadway_tower_squashed.jpg \
    ./seam-carving-dp/images/Broadway_tower_squashed.jpg
++ echo -e '\033[32mImages are the same\033[0m'
Images are the same