Don't track variations at default location (#38)

For variable fonts, when all normalized coordinates are equal to 0, that is equivalent to the default location in variation space and all variation processing can be disabled.

Detect this when pushing a new shaped run and don't store the unnecessary coordinates.

src/layout/data.rs

@@ -248,7 +248,10 @@ impl<B: Brush> LayoutData<B> {
         let metrics = shaper.metrics();
         let cluster_range = self.clusters.len()..self.clusters.len();
         let coords_start = self.coords.len();
-        self.coords.extend_from_slice(shaper.normalized_coords());
+        let coords = shaper.normalized_coords();
+        if coords.iter().any(|coord| *coord != 0) {
+            self.coords.extend_from_slice(coords);
+        }
         let coords_end = self.coords.len();
         let mut run = RunData {
             font_index,