layout: avoid panic due to subtract with overflow

This can occur when 2 Press/Release attached to an HoldTap action are
handled between 2 ticks.

```
ERROR panicked at /home/boris/stuff/kb/keyberon/src/layout.rs:319:32:
attempt to subtract with overflow
```

src/layout.rs

@@ -295,7 +295,7 @@ impl<T, K> WaitingState<T, K> {
             .iter()
             .find(|s| self.is_corresponding_release(&s.event))
         {
-            if self.timeout >= self.delay - since {
+            if self.timeout + since >= self.delay {
                 Some(WaitingAction::Tap)
             } else {
                 Some(WaitingAction::Hold)