Thumbnail basic filter - Create thumbnail only if both requested dimensions are smaller than original image dimensions

Without this change "Imagine\Filter\Basic\Thumbnail" creates thumbnail for dimensions which are bigger than dimensions of original image. Then we will get thumb which is bigger than original image, but quality is much more worst.

Thumbnail is reduced-size versions of pictures, so its main definition is not satisfied.

This change will compare requested and original dimensions, and if both requested dimensions are bigger than original dimensions it will return original image.