The existing implementation has calculated pixels per byte for each pixel.
reduce the calculation of pixels per byte.

name                        old time/op    new time/op     delta
EncodeGray-4                  2.16ms ± 1%     2.16ms ± 1%    -0.28%  (p=0.000 n=86+84)
EncodeGrayWithBufferPool-4    1.99ms ± 0%     1.97ms ± 0%    -0.72%  (p=0.000 n=97+92)
EncodeNRGBOpaque-4            6.51ms ± 1%     6.48ms ± 1%    -0.45%  (p=0.000 n=90+85)
EncodeNRGBA-4                 7.33ms ± 1%     7.28ms ± 0%    -0.69%  (p=0.000 n=89+87)
EncodePaletted-4              5.10ms ± 1%     2.29ms ± 0%   -55.11%  (p=0.000 n=90+85)
EncodeRGBOpaque-4             6.51ms ± 1%     6.51ms ± 0%      ~     (p=0.311 n=94+88)
EncodeRGBA-4                  24.3ms ± 2%     24.1ms ± 1%    -0.87%  (p=0.000 n=91+91)

name                        old speed      new speed       delta
EncodeGray-4                 142MB/s ± 1%    143MB/s ± 1%    +0.26%  (p=0.000 n=86+85)
EncodeGrayWithBufferPool-4   154MB/s ± 0%    156MB/s ± 0%    +0.73%  (p=0.000 n=97+92)
EncodeNRGBOpaque-4           189MB/s ± 1%    190MB/s ± 1%    +0.44%  (p=0.000 n=90+86)
EncodeNRGBA-4                168MB/s ± 1%    169MB/s ± 0%    +0.69%  (p=0.000 n=89+87)
EncodePaletted-4            60.3MB/s ± 1%  134.2MB/s ± 0%  +122.74%  (p=0.000 n=90+85)
EncodeRGBOpaque-4            189MB/s ± 1%    189MB/s ± 0%      ~     (p=0.326 n=94+88)
EncodeRGBA-4                50.6MB/s ± 2%   51.1MB/s ± 1%    +0.87%  (p=0.000 n=91+91)

name                        old alloc/op   new alloc/op    delta
EncodeGray-4                   852kB ± 0%      852kB ± 0%    +0.00%  (p=0.000 n=100+100)
EncodeGrayWithBufferPool-4    1.49kB ± 2%     1.47kB ± 1%    -0.88%  (p=0.000 n=95+90)
EncodeNRGBOpaque-4             860kB ± 0%      860kB ± 0%    +0.00%  (p=0.003 n=98+58)
EncodeNRGBA-4                  864kB ± 0%      864kB ± 0%    +0.00%  (p=0.021 n=100+99)
EncodePaletted-4               849kB ± 0%      849kB ± 0%    +0.00%  (p=0.040 n=100+100)
EncodeRGBOpaque-4              860kB ± 0%      860kB ± 0%      ~     (p=0.062 n=66+98)
EncodeRGBA-4                  3.32MB ± 0%     3.32MB ± 0%    -0.00%  (p=0.044 n=99+99)

name                        old allocs/op  new allocs/op   delta
EncodeGray-4                    32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodeGrayWithBufferPool-4      3.00 ± 0%       3.00 ± 0%      ~     (all equal)
EncodeNRGBOpaque-4              32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodeNRGBA-4                   32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodePaletted-4                36.0 ± 0%       36.0 ± 0%      ~     (all equal)
EncodeRGBOpaque-4               32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodeRGBA-4                    614k ± 0%       614k ± 0%      ~     (all equal)

Change-Id: I716bde2dc80d8111d75d3d765fc09223f770d5a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/187417Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>