SlideImage.size() and SlideImage.get_scaled_size() doesn't honor slide bounds while computing the size.

[AjeyPaiK]

When we initialize a SlideImage instance, the .size property and the .get_scaled_size() member function doesn't help in determining the effective size of the whole slide image if the user chose to use slide_bounds property. This leads to errors in downstream processing currently which can be fixed with extra lines of code. It would be nice if we had a property within SlideImage that could give us the bounded size of the slide.