thumb: scaling factor for requested thumbnails is incorrect

[mangtronix]

The scaling factor requested for thumbnails may be incorrect.

E.g.
1. Load http://www-mang.archive.org/stream/bokeofsaintalban00bernuoft#page/65/mode/thumb
2. Look at the Safari activity window
3. See URLs such as http://ia341221.us.archive.org:80/~mang/BookReader/BookReaderImages.php?zip=/3/items/bokeofsaintalban00bernuoft/bokeofsaintalban00bernuoft_jp2.zip&file=bokeofsaintalban00bernuoft_jp2/bokeofsaintalban00bernuoft_0171.jp2&scale=-236&rotate=0

Expected:
The scaling factor should not be negative and should be the next larger scaling factor compared to the needed image size.

[mangtronix (mang)]

Was asking for negative width image due to improper calculation of page view div width. See https://bugs.edge.launchpad.net/bookreader/+bug/549170

[mangtronix (mang)]

Please refer to the initial description for observed/expected. In Safari if you right-click on a thumbnail image you can select "Open Image In New Window" to see the thumbnail image without any browser scaling. The image dimensions should roughly match what is displayed in the browser window (e.g. not massively larger)

[Winnie (winnie-archive)]

Checked in Safari. No negative scaling.