Fix OS X modifier flags cache getting out of date

Fixes #566.
Closes #567.

README.md

@@ -104,6 +104,7 @@ does not find Doxygen, the documentation will not be generated.
  - [Cocoa] Removed support for OS X 10.6
  - [Cocoa] Bugfix: Full screen windows on secondary monitors were mispositioned
  - [Cocoa] Bugfix: Connecting a joystick that reports no name would segfault
+ - [Cocoa] Bugfix: Modifier flags cache was not updated when window became key
  - [X11] Bugfix: Monitor connection and disconnection events were not reported
  - [X11] Bugfix: Decoding of UTF-8 text from XIM could continue past the end
  - [X11] Bugfix: An XKB structure was leaked during `glfwInit`

src/cocoa_window.m

@@ -206,6 +206,9 @@ static int translateKey(unsigned int key)
 
 - (void)windowDidBecomeKey:(NSNotification *)notification
 {
+    window->ns.modifierFlags =
+        [NSEvent modifierFlags] & NSDeviceIndependentModifierFlagsMask;
+
     if (_glfw.cursorWindow == window &&
         window->cursorMode == GLFW_CURSOR_DISABLED)
     {