Added in preparation for looking at some optimizations around bounds
checks.

BenchmarkAt/rgba-8            100000000     18.5 ns/op      4 B/op   1 allocs/op
BenchmarkAt/rgba64-8          100000000     22.9 ns/op      8 B/op   1 allocs/op
BenchmarkAt/nrgba-8           100000000     18.8 ns/op      4 B/op   1 allocs/op
BenchmarkAt/nrgba64-8         100000000     22.1 ns/op      8 B/op   1 allocs/op
BenchmarkAt/alpha-8           100000000     14.6 ns/op      1 B/op   1 allocs/op
BenchmarkAt/alpha16-8         200000000     6.46 ns/op      0 B/op   0 allocs/op
BenchmarkAt/gray-8            100000000     14.3 ns/op      1 B/op   1 allocs/op
BenchmarkAt/gray16-8          200000000     6.45 ns/op      0 B/op   0 allocs/op
BenchmarkAt/paletted-8        300000000     4.28 ns/op      0 B/op   0 allocs/op
BenchmarkSet/rgba-8           50000000      39.2 ns/op      8 B/op   2 allocs/op
BenchmarkSet/rgba64-8         30000000      45.8 ns/op     16 B/op   2 allocs/op
BenchmarkSet/nrgba-8          50000000      39.3 ns/op      8 B/op   2 allocs/op
BenchmarkSet/nrgba64-8        30000000      45.6 ns/op     16 B/op   2 allocs/op
BenchmarkSet/alpha-8          50000000      34.5 ns/op      2 B/op   2 allocs/op
BenchmarkSet/alpha16-8        50000000      34.9 ns/op      4 B/op   2 allocs/op
BenchmarkSet/gray-8           100000000     20.3 ns/op      1 B/op   1 allocs/op
BenchmarkSet/gray16-8         50000000      36.2 ns/op      4 B/op   2 allocs/op
BenchmarkSet/paletted-8       50000000      39.5 ns/op      1 B/op   1 allocs/op
BenchmarkRGBAAt-8             500000000     3.74 ns/op
BenchmarkRGBASetRGBA-8        300000000     4.33 ns/op
BenchmarkRGBA64At-8           300000000     5.06 ns/op
BenchmarkRGBA64SetRGBA64-8    200000000     6.61 ns/op
BenchmarkNRGBAAt-8            500000000     3.69 ns/op
BenchmarkNRGBASetNRGBA-8      300000000     4.06 ns/op
BenchmarkNRGBA64At-8          300000000     4.98 ns/op
BenchmarkNRGBA64SetNRGBA64-8  200000000     6.62 ns/op
BenchmarkAlphaAt-8            2000000000    1.43 ns/op
BenchmarkAlphaSetAlpha-8      2000000000    1.55 ns/op
BenchmarkAlpha16At-8          1000000000    2.87 ns/op
BenchmarkAlphaSetAlpha16-8    500000000     3.27 ns/op
BenchmarkGrayAt-8             2000000000    1.43 ns/op
BenchmarkGraySetGray-8        2000000000    1.55 ns/op
BenchmarkGray16At-8           1000000000    2.87 ns/op
BenchmarkGraySetGray16-8      500000000     3.14 ns/op

Updates #14884

Change-Id: I349fb214ee75f13ecbc62ac22a40e3b337648f60
Reviewed-on: https://go-review.googlesource.com/136796Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>