New API: Point::is_valid

Cf. MFEK/stroke#21 (rationale, we'd catch `NaN` bugs earlier if we
refused to write at all)
Cf. linebender/norad#198 (similar problem in a competing/featurewise
inferior library)

src/glif/write.rs

@@ -115,6 +115,7 @@ pub fn write_ufo_glif_data<PD: PointData>(glif: &Glif<PD>) -> Result<Vec<u8>, Gl
                     }
 
                     let mut point_node = xmltree::Element::new("point");
+                    debug_assert!(point.is_valid(None));
                     point_node.attributes.insert("x".to_owned(), point.x.to_string());
                     point_node.attributes.insert("y".to_owned(), point.y.to_string());
 

src/point.rs

@@ -104,6 +104,36 @@ pub struct Point<PD: PointData> {
     pub data: Option<PD>,
 }
 
+impl<PD: PointData> Point<PD> {
+    /// `validate_data` parameter allows you to define an `is_valid` (or whatever) impl on your
+    /// `PointData` struct's. You can then pass the function while validating the point as e.g.
+    /// `Some(MyPointData::is_valid)`. It takes an `Option<&PD>` so that you have the choice as to
+    /// whether it's valid or not for your type not to be defined; `Point.data` should probably not
+    /// be defined as an Option<PD>, but removing that's TODO. This API will change when that does
+    /// and should be considered unstable/testing.
+    pub fn is_valid(&self, validate_data: Option<fn(Option<&PD>)->bool>) -> bool {
+        if let Some(validate_point_data_function) = validate_data {
+            if !validate_point_data_function(self.data.as_ref()) {
+                return false
+            }
+        }
+        if self.ptype == PointType::Undefined {
+            return false
+        }
+        if self.x.is_subnormal() || self.y.is_subnormal() {
+            return false
+        }
+        for handle in [self.handle(WhichHandle::A), self.handle(WhichHandle::B)] {
+            if let Handle::At(hx, hy) = handle {
+                if hx.is_subnormal() || hy.is_subnormal() {
+                    return false
+                }
+            }
+        }
+        true
+    }
+}
+
 /// For use by ``Point::handle_or_colocated``
 /// TODO: Replace with Option<WhichHandle>
 #[cfg_attr(feature = "glifserde", derive(Serialize, Deserialize))]