Fix selection of fallback fonts. Fixes monospace fonts in rust guide.

Previously, any font template that could not be found would return
an item from the last resort font family. Now, the last resort font
family is only searched if the entire list of supplied font families
is empty.
auto merge of #3663 : glennw/servo/font-fallback, r=pcwalton

Previously, any font template that could not be found would return an item from the last resort font family. Now, the last resort font family is only searched if the entire list of supplied font families is empty.