Fix loading raw data from .ttc files on macos (#38753)

# Objective

Ensure that functionality which uses the raw font data (such as
rendering text to canvas) works correctly on macOS when the specified
font is a system font that lives in an OpenType Collection (`.ttc`)
file.

## Changes made

- The `read_data_from_file` in each backend now returns a `index: u32`
in addition to `data: Vec<u8>`
- The `data` field on the `Font` type has been renamed to `raw` and the
`data` method on the `Font` type has been renamed to `raw_font`. This
allows the index to be cached as computing is moderately expensive on
macOS (on the order of 100 microseconds).
- Both of the above now store/return a `struct RawFont` instead of a
`FontData` where `RawFont` is defined as `struct RawFont { data:
FontData, index: u32 }`.
- The users of the `data` method have been updated to use the cached
index from `data` rather than calling `.index()` each time.

---------

Signed-off-by: Nico Burns <nico@nicoburns.com>

components/fonts/platform/windows/font_list.rs

@@ -15,8 +15,8 @@ use style::values::specified::font::FontStretchKeyword;
 use webrender_api::NativeFontHandle;
 
 use crate::{
-    EmojiPresentationPreference, FallbackFontSelectionOptions, FontIdentifier, FontTemplate,
-    FontTemplateDescriptor, LowercaseFontFamilyName,
+    EmojiPresentationPreference, FallbackFontSelectionOptions, FontData, FontDataAndIndex,
+    FontIdentifier, FontTemplate, FontTemplateDescriptor, LowercaseFontFamilyName,
 };
 
 pub fn for_each_available_family<F>(mut callback: F)
@@ -67,14 +67,19 @@ impl LocalFontIdentifier {
         }
     }
 
-    pub(crate) fn read_data_from_file(&self) -> Option<Vec<u8>> {
+    pub(crate) fn font_data_and_index(&self) -> Option<FontDataAndIndex> {
         let font = FontCollection::system()
             .font_from_descriptor(&self.font_descriptor)
             .ok()??;
         let face = font.create_font_face();
+        let index = face.get_index();
         let files = face.get_files();
         assert!(!files.is_empty());
-        Some(files[0].get_font_file_bytes())
+
+        let data = files[0].get_font_file_bytes();
+        let data = FontData::from_bytes(&data);
+
+        Some(FontDataAndIndex { data, index })
     }
 }