GN: dm

GN: dm

This builds, links, and runs on Linux.  Have not tried Mac.

I've tested is_debug={true,false} and is_component_build.
It's neat that the component build DM works, but it's also an indication I've missed an essential flag or two... it shouldn't work. :)

The GPU backend isn't working yet, but all the software configurations I've tried look good.

This fleshes out all the other parts of SkCodec too... I noticed we weren't able to decode gifs or webp.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2188643002

Review-Url: https://codereview.chromium.org/2188643002