drawPaletted has to discover R,G,B,A color values of each source image
pixel in a given rectangle. Doing that by calling image.Image.At()
method returning color.Color interface is quite taxing allocation-wise
since interface values go through heap. Introduce special cases for some
concrete source types by fetching color values using type-specific
methods.

name        old time/op    new time/op    delta
Paletted-4    7.62ms ± 4%    3.72ms ± 3%   -51.20%  (p=0.008 n=5+5)

name        old alloc/op   new alloc/op   delta
Paletted-4     480kB ± 0%       0kB ± 0%   -99.99%  (p=0.000 n=4+5)

name        old allocs/op  new allocs/op  delta
Paletted-4      120k ± 0%        0k ± 0%  -100.00%  (p=0.008 n=5+5)

Updates #15759.

Change-Id: I0ce1770ff600ac80599541aaad4c2c826855c8fb
Reviewed-on: https://go-review.googlesource.com/72370Reviewed-by: Nigel Tao <nigeltao@golang.org>