GN: fixes for Mac

 - Make fiddle build on Mac (skipping GL).
 - Now that we're building in SkCodec, we depend on libpng and libjpeg-turbo unconditionally, not just on Linux.
 - Re-arrange third_party a bit so that our targets are Fuchsia/Chrome compatible.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2184133002

NOTREECHECKS=true
This doesn't affect Chrome/Blink, so landing through the closed tree.

Review-Url: https://codereview.chromium.org/2184133002
6 files changed