Explicitly invoke Au::clamp() to avoid colliding with the unstable …

…Rust library method of the

same name.

Fixes breakage from rust-lang/rust#44095.

src/app_unit.rs

@@ -189,7 +189,7 @@ impl Au {
 
     #[inline]
     fn clamp_self(&mut self) {
-        *self = self.clamp()
+        *self = Au::clamp(*self)
     }
 
     #[inline]