Fix for issue #7766
Fix for issue #7766

Add initialization for OS_OSX.mouse_mode in OS_OSX::OS_OSX().  mouse_mode now defaults to OS::MOUSE_MODE_VISIBLE.
(cherry picked from commit 6921e118058ab193f4dfc09ad42f27f5261d7d15)

pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1