SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
With this patch it is possible to create PNG files. Only minimal options
are supported. The PNG is created with one big IDAT chunk containing
only non-compressible DEFLATE blocks.
