benchmark                 old ns/op     new ns/op     delta
BenchmarkDrawString-8     8618239       8149527       -5.44%

This tracks an upstream change in the x/exp/shiny/font package.

This is more consistent with the standard library's image/color package.

The benchmarks didn't seem to change significantly.

The slice Foos is plural; it refers to multiple elements. The element Foos[i]
is the i'th Foo.

The previous "the endpoints are inclusive" comment seems confusing. It's true
that the bounding box's max X equals the right-most coordinate, which suggests
<= instead of <, but that node's coordinate is itself exclusive. Consider the
solid 1-pixel square: (0, 0), (64, 0), (64, 64), (0, 64) in fixed.Point26_6
coordinates. The right-most coordinate is 64, and the bounding box's max X
equals 64, but rasterizing that square only affects sub-pixels up to but not
including 64.

Instead, it seems accurate to follow the fixed.Rectangle26_6 description, in
that the max values are exclusive.

benchmark                 old ns/op     new ns/op     delta
BenchmarkDrawString-4     88103151      14188817      -83.90%

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.