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 *
This fixes a bug where the application would crash if the user
changed the default values for opacity or visibility of a layer
and then tried to draw on it.
The previous names (RGBA32 and RGB32) were misleading since that's not
the actual byte order in memory. The new names reflect exactly how the
color values get laid out in bitmap data.
