[read-fonts] impl From<VariationIdx> for DeltaSetIdx

This is a tiny quality-of-life improvement, since this conversion is
necessary in order to get the variation data out of the data store.

This also derives the standard traits for DeltaSetIndex.

read-fonts/src/tables/layout.rs

@@ -7,6 +7,8 @@ use core::cmp::Ordering;
 
 pub use lookupflag::LookupFlag;
 
+use super::variations::DeltaSetIndex;
+
 #[cfg(test)]
 #[path = "../tests/layout.rs"]
 mod spec_tests;
@@ -350,6 +352,15 @@ fn iter_packed_values(raw: u16, format: DeltaFormat, n: usize) -> impl Iterator<
     decoded.into_iter().flatten()
 }
 
+impl From<VariationIndex<'_>> for DeltaSetIndex {
+    fn from(src: VariationIndex) -> DeltaSetIndex {
+        DeltaSetIndex {
+            outer: src.delta_set_outer_index(),
+            inner: src.delta_set_inner_index(),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

read-fonts/src/tables/variations.rs

@@ -7,7 +7,7 @@ use super::gvar::SharedTuples;
 use std::iter::Skip;
 
 /// Outer and inner indices for reading from an [ItemVariationStore].
-#[derive(Copy, Clone, Debug)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct DeltaSetIndex {
     /// Outer delta set index.
     pub outer: u16,