Serve hinted fonts to Windows Desktop only

One of the performance issues in Windows Phones may have been caused by the non hinted version of the fonts, that also happen to be served in a rather blunt way.

This PR restricts hinted fonts to Desktop windows only.

Are excluded:
- Windows phone
- Windows phone in desktop mode

See http://stackoverflow.com/questions/9926504/how-do-i-check-windows-phone-useragent-with-javascript for the bit about the WPDesktop user agent.

Related to #2127

![ ](http://media.tumblr.com/7ef0237c00bbd397c5ac392375d212bd/tumblr_inline_muzu5lCkAo1raprkq.gif)