poor quality of small thumbnails

[rmcloughlin]

OpenSlide.createThumbnailImage(size) creates poor-quality images when the desired size is significantly smaller than the smallest layer included in the slide.

For example we have a slide with three layers. The sizes of the layers are approximately:

54000 x 43000
13500 x 10750
 3375 x  2688

When I create a size 300 thumbnail (with OpenSlide.createThumbnailImage(300)) it is noticeably grainy. Note that this is not the case with the python API (get_thumbnail), which creates a nice looking thumbnail of that size from the same slide.