Fix keys filtered by IME being passed to callback

Closes #651.

src/win32_window.c

@@ -329,6 +329,13 @@ static int translateKey(WPARAM wParam, LPARAM lParam)
         return GLFW_KEY_LEFT_CONTROL;
     }
 
+    if (wParam == VK_PROCESSKEY)
+    {
+        // IME notifies that keys have been filtered by setting the virtual
+        // key-code to VK_PROCESSKEY
+        return _GLFW_KEY_INVALID;
+    }
+
     return _glfw.win32.publicKeys[HIWORD(lParam) & 0x1FF];
 }