Intent to prototype and ship: EXIF image density correction.

[Emilio Cobos Álvarez]

Summary: Honor EXIF metadata that declares the pixel density of the
image. Other browsers do this, and it seems useful.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1680387

Standard: https://github.com/whatwg/html/pull/5574

Platform coverage: all

Preference: image.exif-density-correction.enabled

DevTools bug: N/A, I think

Other browsers: This one is fun. Even though the spec PR didn't land,
and I didn't see any intent email on blink-dev@ with regards to this (I
might have missed it though? not sure) this is shipped on stable
versions of Chromium and Safari (note that non-Apple WebKit ports still
lack support for this though):

*
https://chromium.googlesource.com/chromium/src/+/86ae1272b4a60b42439bc115777bb916fa3a6720
* https://trac.webkit.org/changeset/268886/webkit

web-platform-tests: There are a few in
https://wpt.fyi/results/density-size-correction, but there were tests
missing for some important cases (like interaction with image-set /
srcset) which I'm adding tests for.

-- Emilio