Only apply avar table to the variation axis being set (#175)

Fixes harfbuzz/rustybuzz#136

src/lib.rs

@@ -2323,13 +2323,11 @@ impl<'a> Face<'a> {
         for (i, var_axis) in self.variation_axes().into_iter().enumerate() {
             if var_axis.tag == axis {
                 self.coordinates.data[i] = var_axis.normalized_value(value);
-            }
-        }
 
-        // TODO: optimize
-        if let Some(avar) = self.tables.avar {
-            // Ignore error.
-            let _ = avar.map_coordinates(self.coordinates.as_mut_slice());
+                if let Some(avar) = self.tables.avar {
+                    let _ = avar.map_coordinate(self.coordinates.as_mut_slice(), i);
+                }
+            }
         }
 
         Some(())

src/tables/avar.rs

@@ -114,13 +114,22 @@ impl<'a> Table<'a> {
         })
     }
 
-    /// Maps coordinates.
-    pub fn map_coordinates(&self, coordinates: &mut [NormalizedCoordinate]) -> Option<()> {
+    /// Maps a single coordinate
+    pub fn map_coordinate(
+        &self,
+        coordinates: &mut [NormalizedCoordinate],
+        coordinate_index: usize,
+    ) -> Option<()> {
         if usize::from(self.segment_maps.count) != coordinates.len() {
             return None;
         }
 
-        for (map, coord) in self.segment_maps.into_iter().zip(coordinates) {
+        if let Some((map, coord)) = self
+            .segment_maps
+            .into_iter()
+            .zip(coordinates)
+            .nth(coordinate_index)
+        {
             *coord = NormalizedCoordinate::from(map_value(&map, coord.0)?);
         }