Khmer bold fonts

[pnorman]

#358 (comment) recommended the Khmer OS Kangrey for bold Khmer text, but I deferred because it wasn't packaged on Ubuntu, there are much bigger issues with Khmer text until we move to a version of Mapnik with Harfbuzz support (#358 (comment)) , and there is currently one Khmer label on the map, which doesn't use bold.

Once other font work is done, I want to revisit this, as I should have a better grasp on if other non-packaged fonts would be an improvement.