In previous versions what has been stored/specified as perceptual blending or
compositing spaces has really been the non-linear variant of the images babl
space.
To maintain loading of old files, the code has been updated to actually mean
non-linear and a new perceptual value has been added to the GimpLayerColorSpace
enum, while preserving all old enum values.
This change bumps XCF file version to 23
3f3b29ba12