Fix crash launching landscape Windows 8 app in portrait
Fixes issue #1136
Calculate the back buffer scale in terms of the same orientation as the preferred back buffer size.  Avoids crazy scales such as (1.7, 0.5) causing back buffers of 2430x432 and "Not enough storage" exceptions.
Merge pull request #1137 from slygamer/back-buffer-scale

Fix crash launching landscape Windows 8 app in portrait