LoadClosestFont loads a Font.
You pass the properties of the ideal font you want to load in the FontDescriptor you pass to this function. If the requested font is not available on the system, the closest matching font is used. This means that, for instance, if you specify a Weight of TextWeightUltraHeavy and the heaviest weight available for the chosen font family is actually TextWeightBold, that will be used instead. The specific details of font matching beyond this description are implementation defined. This also means that getting a descriptor back out of a Font may return a different desriptor.
TODO guarantee that passing *that* back into LoadClosestFont() returns the same font