Fix System Control rollover bug

Previously, rolling over from one System Control key to another would
cause the second one to be released as soon as the first one was
released, because the empty release report would be sent
unconditionally on release of any System Control key.

This change stores the value of the last System Control key
pressed. When a System Control key is released, it first checks to see
that the released key's keycode matches the last one pressed before
sending the empty report.

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

src/kaleidoscope/driver/hid/base/Keyboard.h

@@ -207,11 +207,16 @@ class Keyboard {
     consumer_control_.release(CONSUMER(mapped_key));
   }
   void pressSystemControl(Key mapped_key) {
-    system_control_.press(mapped_key.getKeyCode());
+    uint8_t keycode = mapped_key.getKeyCode();
+    system_control_.press(keycode);
+    last_system_control_keycode_ = keycode;
   }
   void releaseSystemControl(Key mapped_key) {
+    uint8_t keycode = mapped_key.getKeyCode();
+    if (keycode == last_system_control_keycode_) {
       system_control_.release();
     }
+  }
 
   // pressKey takes a Key, as well as optional boolean 'toggledOn' which defaults
   // to 'true'
@@ -373,6 +378,12 @@ class Keyboard {
   }
 
  private:
+  // To prevent premature release of a System Control key when rolling
+  // over from one to another, we record the last System Control
+  // keycode that was pressed. It's initialized to zero, which should
+  // not be a valid System Control keycode.
+  uint8_t last_system_control_keycode_ = 0;
+
   // modifier_flag_mask is a bitmask of modifiers that we found attached to
   // keys that were newly pressed down during the most recent cycle with any new
   // keypresses.