vidarh/skrift: A pure Ruby conversion (*not* wrapper) of the libschrift TrueType font renderer

Skrift rasterises TrueType/OpenType outlines to 8-bit anti-aliased coverage, in pure Ruby with no native extensions. If you need performance and don't mind a C dependency, use libschrift; if you want pure Ruby — and are happy to cache glyphs after rendering — this is for you. "Skrift" is Norwegian for "text" / "writing" / "scripture", a close cognate of "Schrift" — chosen to set this apart from libschrift and leave that name free for a direct C wrapper.