Add render in BG_MODE_4

[gallottino]

Documented here: BG_MODE_4

In this PR I also added a macro to simplify lock syntax:

acquire_lock!(lock_var, acquired => { you can use acquired here } );

It was usefull because sometime we have to acquire a lock only in the needed scopes. In this way it is more cleaner to use a mutex more times in the same flow.

For example assume you have two function fn_1(), fn_2()

fn_1() {
  // it needs a lock
  let acquired_var = self.lock_var.lock().unwrap();
  acquired_var.do_something();

 fn_2();
}

fn_2() {
  // it needs a lock
  let acquired_var = self.lock_var.lock().unwrap();
  acquired_var.do_other_stuff();
}

if you call fn_1() a deadlock occurs.

with this macro you can do this without problems:

fn_1() {
  // it needs a lock
  acquire_lock!(self.lock_var, var => { var.do_something(); });
 fn_2();
}

With just test_mode_4 path/rom.gba you can check the correct behavior with the help of PaletteVisualizer.