tjGetScaledSize() would never be able to accommodate scaling factors > 1, so replace...

tjGetScaledSize() would never be able to  accommodate scaling factors > 1, so replace it with a function that returns a list of fractional scaling factors that TurboJPEG supports.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@477 632fc199-4ca6-4c93-a231-07263d6284db