bmp: optimize decoding and encoding 0xH sized images.

It should be quick regardless of how large H is.

Fixes golang/go#10746

Change-Id: Icde36047e88d9786e64f44724b7ba8b38db2a33f
Reviewed-on: https://go-review.googlesource.com/9836
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed