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/shared/gfx/lib.rs

@@ -2,71 +2,11 @@
  * 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/. */
 
-#![crate_name = "gfx_traits"]
-#![crate_type = "rlib"]
 #![deny(unsafe_code)]
 
-pub mod print_tree;
-
-use std::sync::atomic::{AtomicU64, Ordering};
-use std::sync::Arc;
-
 use malloc_size_of_derive::MallocSizeOf;
 use range::{int_range_index, RangeIndex};
 use serde::{Deserialize, Serialize};
-use webrender_api::{
-    Epoch as WebRenderEpoch, FontInstanceFlags, FontInstanceKey, FontKey, NativeFontHandle,
-};
-
-/// A newtype struct for denoting the age of messages; prevents race conditions.
-#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, Ord, PartialEq, PartialOrd, Serialize)]
-pub struct Epoch(pub u32);
-
-impl Epoch {
-    pub fn next(&mut self) {
-        self.0 += 1;
-    }
-}
-
-impl From<Epoch> for WebRenderEpoch {
-    fn from(val: Epoch) -> Self {
-        WebRenderEpoch(val.0)
-    }
-}
-
-pub trait WebRenderEpochToU16 {
-    fn as_u16(&self) -> u16;
-}
-
-impl WebRenderEpochToU16 for WebRenderEpoch {
-    /// The value of this [`Epoch`] as a u16 value. Note that if this Epoch's
-    /// value is more than u16::MAX, then the return value will be modulo
-    /// u16::MAX.
-    fn as_u16(&self) -> u16 {
-        (self.0 % u16::MAX as u32) as u16
-    }
-}
-
-/// 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)
-    }
-}
 
 int_range_index! {
     #[derive(Deserialize, MallocSizeOf, Serialize)]
@@ -74,61 +14,3 @@ int_range_index! {
     /// the middle of a glyph.
     struct ByteIndex(isize)
 }
-
-/// The type of fragment that a scroll root is created for.
-///
-/// This can only ever grow to maximum 4 entries. That's because we cram the value of this enum
-/// into the lower 2 bits of the `ScrollRootId`, which otherwise contains a 32-bit-aligned
-/// heap address.
-#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
-pub enum FragmentType {
-    /// A StackingContext for the fragment body itself.
-    FragmentBody,
-    /// A StackingContext created to contain ::before pseudo-element content.
-    BeforePseudoContent,
-    /// A StackingContext created to contain ::after pseudo-element content.
-    AfterPseudoContent,
-}
-
-/// The next ID that will be used for a special scroll root id.
-///
-/// A special scroll root is a scroll root that is created for generated content.
-static NEXT_SPECIAL_SCROLL_ROOT_ID: AtomicU64 = AtomicU64::new(0);
-
-/// If none of the bits outside this mask are set, the scroll root is a special scroll root.
-/// Note that we assume that the top 16 bits of the address space are unused on the platform.
-const SPECIAL_SCROLL_ROOT_ID_MASK: u64 = 0xffff;
-
-/// Returns a new scroll root ID for a scroll root.
-fn next_special_id() -> u64 {
-    // We shift this left by 2 to make room for the fragment type ID.
-    ((NEXT_SPECIAL_SCROLL_ROOT_ID.fetch_add(1, Ordering::SeqCst) + 1) << 2) &
-        SPECIAL_SCROLL_ROOT_ID_MASK
-}
-
-pub fn combine_id_with_fragment_type(id: usize, fragment_type: FragmentType) -> u64 {
-    debug_assert_eq!(id & (fragment_type as usize), 0);
-    if fragment_type == FragmentType::FragmentBody {
-        id as u64
-    } else {
-        next_special_id() | (fragment_type as u64)
-    }
-}
-
-pub fn node_id_from_scroll_id(id: usize) -> Option<usize> {
-    if (id as u64 & !SPECIAL_SCROLL_ROOT_ID_MASK) != 0 {
-        return Some(id & !3);
-    }
-    None
-}
-
-pub trait WebrenderApi {
-    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;
-}