image/gif: add more writer benchmarks
image/gif: add more writer benchmarks

The two existing benchmarks encode randomized pixels, which isn't very
representative. The two new benchmarks encode a PNG photo as a GIF.

Also rename the benchmarks for consistency.

Also fix the bytes-per-op measure for paletted images, which are 1 (not
4) bytes per pixel.

Also simplify BenchmarkEncodeRandomPaletted (formerly just called
BenchmarkEncode). It doesn't need to generate a random palette (and the
GIF encoder largely doesn't care about the palette's RGBA values).
Use palette.Plan9 instead, a pre-existing 256-element color palette.

Change-Id: I10a6ea4e9590bb0d9f76e8cc0f4a88d43b1d650d
Reviewed-on: https://go-review.googlesource.com/c/go/+/248218
Reviewed-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

CAFxX/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,golang/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,cloudflare/go