Move non-gfx things out of `gfx_traits` and create a `base` crate (#32296)

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`.

1 files changed, 15 insertions, 1 deletions

diff --git a/components/shared/webrender/lib.rs b/components/shared/webrender/lib.rs
index a71f9f67760..8af3a17b609 100644
--- a/components/shared/webrender/lib.rs
+++ b/components/shared/webrender/lib.rs
@@ -9,7 +9,10 @@ use std::sync::{Arc, Mutex};
 
 use euclid::default::Size2D;
 use webrender_api::units::TexelRect;
-use webrender_api::{ExternalImage, ExternalImageHandler, ExternalImageId, ExternalImageSource};
+use webrender_api::{
+    ExternalImage, ExternalImageHandler, ExternalImageId, ExternalImageSource, FontInstanceFlags,
+    FontInstanceKey, FontKey, NativeFontHandle,
+};
 
 /// This trait is used as a bridge between the different GL clients
 /// in Servo that handles WebRender ExternalImages and the WebRender
@@ -164,3 +167,14 @@ impl ExternalImageHandler for WebrenderExternalImageHandlers {
         };
     }
 }
+
+pub trait WebRenderFontApi {
+    fn add_font_instance(
+        &self,
+        font_key: FontKey,
+        size: f32,
+        flags: FontInstanceFlags,
+    ) -> FontInstanceKey;
+    fn add_font(&self, data: Arc<Vec<u8>>, index: u32) -> FontKey;
+    fn add_system_font(&self, handle: NativeFontHandle) -> FontKey;
+}