Fix #310 - correct font-size of non-standard dpi

There was some superfluous logic that was incorrectly providing font-size values to pango.

I've fixed the logic, and added a reftest. One reason this existed is that all reftests are run at the default 72dpi. I added a quick check on a new filename convention: simply ending the reftest base file name in -48dpi will cause the file to render at 48dpi instead of 72.