Losslessly optimizes the PNG images in the `resources` folder with:
```
oxipng -o max -a -s
oxipng -o max --zopfli -a -s
```
Tests are unaffected by this commit in order to avoid potential
breakages.
As an aside: shouldn't `tumbeast.png` and `badcert.jpg` be removed? For
`tumbeast.png`, the related page `resources/not-found.html` doesn't seem
to be reachable by any means (the usual dry error message is used
instead, as far as my testing goes, neither `about:not-found` nor
`servo:not-found` work), and the only use of `badcert.jpg` (in
`badcert.html`) got removed in 8422dac811.
Testing: Unnecessary. Since the optimized images haven't changed their
extension, so long as Servo's PNG parsing is correct, there should be no
change aside from reduced binary sizes.
Signed-off-by: maple! <averyrudelphe@gmail.com>
