Use oxipng instead of optipng

[humdingerb]

I just tried oxipg and due to its multithreadedness it's much quicker and - according to my one test - produces smaller PNGs.

❯ time oxipng -o 4 test.png
Processing: test.png
3249217 bytes (21.95% smaller): test.png

real    0m8,252s
user    0m23,410s
sys     0m0,110s

❯ time optipng -o 6 test.png 
** Processing: test.png
2253x1969 pixels, 4x8 bits/pixel, RGB+alpha
Reducing image to 3x8 bits/pixel, RGB
Input IDAT size = 4156655 bytes
Input file size = 4162796 bytes

Trying:
  zc = 9  zm = 8  zs = 0  f = 1         IDAT size = 3879354
  zc = 9  zm = 8  zs = 0  f = 2         IDAT size = 3660579
  zc = 9  zm = 8  zs = 0  f = 4         IDAT size = 3553183
  zc = 9  zm = 8  zs = 0  f = 5         IDAT size = 3550571
                               
Selecting parameters:
  zc = 9  zm = 8  zs = 0  f = 5         IDAT size = 3550571

Output IDAT size = 3550571 bytes (606084 bytes decrease)
Output file size = 3550628 bytes (612168 bytes = 14.71% decrease)


real    3m44,620s
user    3m44,497s
sys     0m0,096s