Add documentation and improve API

src/font/font.rs

@@ -0,0 +1,27 @@
+use std::{collections::HashMap, sync::Mutex};
+
+use super::{CacheKey, CacheItem};
+
+pub struct Font<'a> {
+    pub info: &'a fontdb::FaceInfo,
+    pub data: &'a [u8],
+    pub index: u32,
+    pub rustybuzz: rustybuzz::Face<'a>,
+    pub swash: swash::FontRef<'a>,
+    pub scale_context: Mutex<swash::scale::ScaleContext>,
+    pub cache: Mutex<HashMap<CacheKey, CacheItem>>,
+}
+
+impl<'a> Font<'a> {
+    pub fn new(info: &'a fontdb::FaceInfo, data: &'a [u8], index: u32) -> Option<Self> {
+        Some(Self {
+            info,
+            data,
+            index,
+            rustybuzz: rustybuzz::Face::from_slice(data, index)?,
+            swash: swash::FontRef::from_index(data, index as usize)?,
+            scale_context: Mutex::new(swash::scale::ScaleContext::new()),
+            cache: Mutex::new(HashMap::new()),
+        })
+    }
+}