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`.

components/layout/display_list/builder.rs

@@ -13,6 +13,7 @@ use std::sync::Arc;
 use std::{f32, mem};
 
 use app_units::{Au, AU_PER_PX};
+use base::id::{BrowsingContextId, PipelineId};
 use bitflags::bitflags;
 use canvas_traits::canvas::{CanvasMsg, FromLayoutMsg};
 use embedder_traits::Cursor;
@@ -21,12 +22,11 @@ use euclid::{rect, SideOffsets2D};
 use fnv::FnvHashMap;
 use gfx::text::glyph::ByteIndex;
 use gfx::text::TextRun;
-use gfx_traits::{combine_id_with_fragment_type, FragmentType, StackingContextId};
 use ipc_channel::ipc;
 use log::{debug, warn};
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image_cache::UsePlaceholder;
 use range::Range;
+use script_layout_interface::{combine_id_with_fragment_type, FragmentType};
 use script_traits::compositor::ScrollSensitivity;
 use servo_config::opts;
 use servo_geometry::{self, MaxRect};
@@ -53,6 +53,7 @@ use webrender_api::{
     NinePatchBorderSource, NormalBorder, PropertyBinding, StickyOffsetBounds,
 };
 
+use super::StackingContextId;
 use crate::block::BlockFlow;
 use crate::context::LayoutContext;
 use crate::display_list::background::{self, get_cyclic};

components/layout/display_list/items.rs

@@ -16,11 +16,10 @@ use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::{f32, fmt};
 
+use base::id::PipelineId;
+use base::print_tree::PrintTree;
 use embedder_traits::Cursor;
 use euclid::{SideOffsets2D, Vector2D};
-use gfx_traits::print_tree::PrintTree;
-use gfx_traits::{self, StackingContextId};
-use msg::constellation_msg::PipelineId;
 use net_traits::image::base::Image;
 use script_traits::compositor::{ScrollSensitivity, ScrollTreeNodeId};
 use serde::Serialize;
@@ -35,6 +34,8 @@ use webrender_api::{
     SpatialId, StickyOffsetBounds, TransformStyle,
 };
 
+use super::StackingContextId;
+
 /// The factor that we multiply the blur radius by in order to inflate the boundaries of display
 /// items that involve a blur. This ensures that the display item boundaries include all the ink.
 pub static BLUR_INFLATION_FACTOR: i32 = 3;

components/layout/display_list/mod.rs

@@ -2,6 +2,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use malloc_size_of_derive::MallocSizeOf;
+use serde::{Deserialize, Serialize};
+
 pub use self::builder::{
     BorderPaintingMode, DisplayListBuildState, IndexableText, StackingContextCollectionFlags,
     StackingContextCollectionState,
@@ -15,3 +18,24 @@ pub(crate) mod conversions;
 mod gradient;
 pub mod items;
 mod webrender_helpers;
+
+/// A unique ID for every stacking context.
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
+pub struct StackingContextId(
+    /// The identifier for this StackingContext, derived from the Flow's memory address
+    /// and fragment type.  As a space optimization, these are combined into a single word.
+    pub u64,
+);
+
+impl StackingContextId {
+    /// Returns the stacking context ID for the outer document/layout root.
+    #[inline]
+    pub fn root() -> StackingContextId {
+        StackingContextId(0)
+    }
+
+    pub fn next(&self) -> StackingContextId {
+        let StackingContextId(id) = *self;
+        StackingContextId(id + 1)
+    }
+}

components/layout/display_list/webrender_helpers.rs

@@ -7,9 +7,9 @@
 //           This might be achieved by sharing types between WR and Servo display lists, or
 //           completely converting layout to directly generate WebRender display lists, for example.
 
-use gfx_traits::WebRenderEpochToU16;
+use base::id::PipelineId;
+use base::WebRenderEpochToU16;
 use log::trace;
-use msg::constellation_msg::PipelineId;
 use script_traits::compositor::{
     CompositorDisplayListInfo, ScrollSensitivity, ScrollTreeNodeId, ScrollableNodeInfo,
 };