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)

platform/osx/os_osx.mm

@@ -1781,7 +1781,8 @@ String OS_OSX::get_joy_guid(int p_device) const {
 OS_OSX* OS_OSX::singleton=NULL;
 
 OS_OSX::OS_OSX() {
-
+	
+	mouse_mode=OS::MOUSE_MODE_VISIBLE;
 	main_loop=NULL;
 	singleton=this;
 	autoreleasePool = [[NSAutoreleasePool alloc] init];