Seperate freetype face and named instance indices (#39213)

In servo, each `LocalFontIdentifier` has an `fn index() -> u32`, which
returns the index of the font within the font file in case it is a font
collection (`.ttc` instead of `.ttf`). The way this index is obtained is
platform-dependent.

On systems using `fontconfig`, we get the index by querying `FC_INDEX`:


d2c78db981/components/fonts/platform/freetype/font_list.rs (L109-L112)

There is a sneaky bug here: In addition to the aforementioned face
index, the value for `FC_INDEX` contains the index of the named instance
of the face in the upper 16 bits. This behaviour is completely
undocumented, but FreeType uses the same format:
https://freetype.org/freetype2/docs/reference/ft2-face_creation.html#ft_open_face.

This wasn't a problem for the longest time, because the only consumer of
the `index` value coming from fontconfig was FreeType. However, sometime
after
29e618dcf7,
we started also passing it to`read-fonts` . `read-fonts` expects only
the face index, causing it to return errors as seen in
https://github.com/servo/servo/issues/39209.

The fix is to seperate the two indices when storing them in a
`LocalFontDescriptor` and pass only the lower 16 bits to `read-fonts`.
I'm unsure whether we should continue passing the named instance index
to FreeType since we don't actually support variable fonts, but I've
opted to keep the current behaviour for now.

Fixes: https://github.com/servo/servo/issues/39209

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/fonts/tests/font_context.rs

@@ -186,7 +186,8 @@ mod font_context {
 
             let local_font_identifier = LocalFontIdentifier {
                 path: path.to_str().expect("Could not load test font").into(),
-                variation_index: 0,
+                face_index: 0,
+                named_instance_index: 0,
             };
             let handle = PlatformFont::new_from_local_font_identifier(
                 local_font_identifier.clone(),