Clear masked keys on release in `Runtime.handleKeyswitchEvent()`

This fixes a problem with a plugin that returns `ABORT` from its
`onKeyswitchEvent()` handler, for a masked key addr.  I'm convinced that a
better solution is to switch from using `Key_NoKey` (a.k.a. `Key_Masked`) to
using `Key_Transparent` as the default for new events, and thus, the value that
signals that a lookup should be done, but this at least fixes the bug for now,
with a much less intrusive change.

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

src/kaleidoscope/Runtime.cpp

@@ -102,6 +102,11 @@ Runtime_::handleKeyswitchEvent(KeyEvent event) {
     // When a key toggles off, set the event's key value to whatever the key's
     // current value is in the live keys state array.
     event.key = live_keys[event.addr];
+    // If that key was masked, unmask it and return.
+    if (event.key == Key_Masked) {
+      live_keys.clear(event.addr);
+      return;
+    }
   } else if (event.key == Key_NoKey) {
     // When a key toggles on, unless the event already has a key value (i.e. we
     // were called by a plugin rather than `actOnMatrixScan()`), we look up the