losslessly minimize png and jpg images (#567)

With ImageOptim v1.9.3 (with zopflipng, pngout, oxipng, advpng,
jpegoptim, jpegtran).

Then a manual pass with:
```sh
oxipng --strip safe --nc --opt 1 --interlace 0 --alpha  # v9.1.5
pngout -k0  # v20230322
zopflipng \
  --lossy_transparent -y \
  --keepchunks=tEXt,zTXt,iTXt,gAMA,sRGB,iCCP,bKGD,pHYs,sBIT,tIME,oFFs,acTL,fcTL,fdAT,prVW,mkBF,mkTS,mkBS,mkBT \
  --filters=0pme  # v1.0.3
advpng -z -4  # v2.6
optipng -o7  # v1.6.50, one final pass, but it did not make a difference
```

Where `--lossy_transparent` isn't visually lossy: "remove colors behind
alpha channel 0. No visual difference, removes hidden information."

Reduces PDF size from 3'800'542 to 3'508'496 bytes.

Filename                          | Before |  After | Gain % |  -bytes |
:-------------------------------- | -----: | -----: | -----: | --------:
proxy-firefox-screenshot.png      | 308507 | 169295 | 45.12% | -139212 |
firefox-copy-as-curl.png          | 300049 | 243905 | 18.71% |  -56144 |
safari-copy-as-curl.png           | 157057 | 103411 | 34.16% |  -53646 |
firefox-cliget.png                | 270362 | 217695 | 19.48% |  -52667 |
wireshark-ssl-master-secret.png   | 118616 |  73986 | 37.63% |  -44630 |
wireshark-screenshot.png          | 147922 | 113012 | 23.60% |  -34910 |
chrome-copy-as-curl.png           | 176290 | 141553 | 19.70% |  -34737 |
slide-http3-backends.jpg          | 165157 | 135704 | 17.83% |  -29453 |
cover.jpg                         | 606479 | 578042 |  4.69% |  -28437 |
release-cycle.jpg                 | 111017 |  84522 | 23.87% |  -26495 |
slide-libcurl-backends.jpg        | 180840 | 155300 | 14.12% |  -25540 |
slide-transfer-state-machine.jpg  | 181980 | 158587 | 12.85% |  -23393 |
curl-runs-in-all-your-devices.jpg | 272816 | 256672 |  5.92% |  -16144 |
slide-protocol-handlers.jpg       |  89313 |  86125 |  3.57% |   -3188 |
20-billion.jpg                    |  95760 |  93785 |  2.06% |   -1975 |
cover_small.jpg                   |  15369 |  14990 |  2.47% |    -379 |
devgraph-800.png                  |  32911 |  32776 |  0.41% |    -135 |
multipart-form.png                |  11600 |  11474 |  1.09% |    -126 |
TOTAL                             |3242045 |2670834 |        | -571211 |

Author: vszakats