Fix Windows spec by using element_dimension instead of normal

* interestingly on Windows it seems like there are margins there..
  for some reason
* I dare you travis to fail me now!

TextFontFactory is still missing from the party... I will take a look at it but not too sure I can fix it.

Fixes #915 
From what I know, this seems ok. Worst case if you somehow managed to ask separately for the same font with 15.75 and 16, they'd return you the same cached font... not the end of the world by any means.