fonts: Improve font fallback (#32286)

- Better detect situations where emoji is necessary by looking ahead one
  character while laying out. This allow processing Unicode presentation
  selectors. When detecting emoji, put emoji fonts at the front of
  fallback lists for all platforms.

  This enables monochrome emoji on Windows. Full-color emoji on Windows
  probably needs full support for processing the COLR table and drawing
  separate glyph color layers.

- Improve the font fallback list on FreeType platforms. Ideally, Servo
  would be able to look through the entire font list to find the best
  font for a certain character, but until that time we can make sure the
  font list contains the "Noto Sans" fonts which cover most situations.

Fixes #31664.
Fixes #12944.

components/gfx/text/mod.rs

@@ -2,6 +2,8 @@
  * 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 unicode_properties::{emoji, UnicodeEmoji};
+
 pub use crate::text::shaping::Shaper;
 pub use crate::text::text_run::TextRun;
 
@@ -9,3 +11,32 @@ pub mod glyph;
 pub mod shaping;
 pub mod text_run;
 pub mod util;
+
+#[derive(Clone, Copy, Debug)]
+pub struct FallbackFontSelectionOptions {
+    pub character: char,
+    pub prefer_emoji_presentation: bool,
+}
+
+impl Default for FallbackFontSelectionOptions {
+    fn default() -> Self {
+        Self {
+            character: ' ',
+            prefer_emoji_presentation: false,
+        }
+    }
+}
+
+impl FallbackFontSelectionOptions {
+    pub fn new(character: char, next_character: Option<char>) -> Self {
+        let prefer_emoji_presentation = match next_character {
+            Some(next_character) if emoji::is_emoji_presentation_selector(next_character) => true,
+            Some(next_character) if emoji::is_text_presentation_selector(next_character) => false,
+            _ => character.is_emoji_char(),
+        };
+        Self {
+            character,
+            prefer_emoji_presentation,
+        }
+    }
+}