PNG_FORMAT_RGB and PNG_COLOR_TYPE_RGB both evaluate to 2, but PNG_FORMAT_RGBA evaluates to 3 while PNG_COLOR_TYPE_RGBA evaluates to 6; the bit indicating a palette is 1 while the bit indicating alpha is 4.
SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
