This is in anticipation of caching glyph images. Quantization means that cache
hits are more likely.

Also make NewFace take an *Options instead of an Options.

benchmark                 old ns/op     new ns/op     delta
BenchmarkDrawString-4     96914389      88683659      -8.49%

benchmark                 old allocs     new allocs     delta
BenchmarkDrawString-4     32287          0              -100.00%

benchmark                 old bytes     new bytes     delta
BenchmarkDrawString-4     1929619       156           -99.99%

Its implementation is mostly a copy/paste of the freetype.Context type.
Follow-up commits will make it more efficient.

Also add an example that uses a truetype.Face and the
golang.org/x/exp/shiny/font package to draw text.