Respect &scale option in width/height attributes

Let's say we have an original image 1000x1000px and want to create a thumb at 250x250 with doubled pixels (retina resolution) so we set `&scale=2`.

If you set the `&dims=1` options to let pthumb add correct widht/height attributes, you'll get 500 and 500px for both attributes. Which are the correct values of the image, but since it should be a retina image those width/height attributes should be 250/250.

So I came up with this little solution for myself, where I check if `&scale` was set and calculate correct values.