Bit packs image data when writing images with fewer than 16
colors in its palette. Reading of bit packed image data was
already implemented.

Fixes #19879

Change-Id: I0a06f9599a163931e20d3503fc3722e5101f0070
Reviewed-on: https://go-review.googlesource.com/134235Reviewed-by: Nigel Tao <nigeltao@golang.org>