Correct label padding to be positive, not negative.

Before, padding of label had to be set to a negative value in order to make the label move inward. This fixes it. I think this behavior was by mistake, not intentionally. To see the problem, run this example before and after the commit:

    from kivy.app import runTouchApp
    from kivy.lang import Builder
    
    
    kv = """
    Label:
        text: "Here is some text"
        text_size: self.size
        padding: [20, 5]
    """
    
    if __name__ == '__main__':
        runTouchApp(Builder.load_string(kv))
    
Before the commit, part of the label moves outside the edges of the window.