Add glfwGetKeyScancode

Allows retrieval of platform scancode from GLFW_KEY_*.  Implemented for
Win32, Cocoa and X11.  Stubs for Mir and Wayland.

Closes #830.

src/cocoa_window.m

@@ -1511,6 +1511,16 @@ const char* _glfwPlatformGetKeyName(int key, int scancode)
     return _glfw.ns.keyName;
 }
 
+const short int _glfwPlatformGetKeyScancode(int key)
+{
+    if(key <= -1 || key >= (sizeof(_glfw.ns.nativeKeys) / sizeof(_glfw.ns.nativeKeys[0])))
+    {
+        return -1;
+    }
+
+    return _glfw.ns.nativeKeys[key];
+}
+
 int _glfwPlatformCreateCursor(_GLFWcursor* cursor,
                               const GLFWimage* image,
                               int xhot, int yhot)