For a long time, `gfx_traits` has held a lot of things unrelated to graphics
and also unrelated to the `gfx` crate (which is mostly about fonts).
This is a cleanup which does a few things:
1. Move non `gfx` crate things out of `gfx_traits`. This is important in
order to prevent dependency cycles with a different integration between
layout, script, and fonts.
2. Rename the `msg` crate to `base`. It didn't really contain anything
to do with messages and instead mostly holds ids, which are used
across many different crates in Servo. This new crate will hold the
*rare* data types that are widely used.
Details:
- All BackgroundHangMonitor-related things from base to a new
`background_hang_monitor_api` crate.
- Moved `TraversalDirection` to `script_traits`
- Moved `Epoch`-related things from `gfx_traits` to `base`.
- Moved `PrintTree` to base. This should be widely useful in Servo.
- Moved `WebrenderApi` from `base` to `webrender_traits` and renamed it
to `WebRenderFontApi`.
This adds support for table rows, columns, rowgroups and colgroups.
There are few additions here:
1. The createion of fragments, which allows script queries and hit
testing to work properly. These fragments are empty as all cells are
still direct descendants of the table fragment.
2. Properly handling size information from tracks and track groups as
well as frustrating rules about reordering rowgroups.
3. Painting a background seemlessly across track groups and groups. This
is a thing that isn't done in legacy layout (nor WebKit)!
Co-authored-by: Oriol Brufau <obrufau@igalia.com>
