LF-40840 Image compression

• Support the 'exact' option in the WEBP dataset. This was already reviewed in https://freya.seequent.com/projects/REARCHITECT/repos/build_scripts/pull-requests/254/overview.
• Build GDAL python wrappers with the -threads argument, which enables releasing the GIL when possible. The GNUmakefile's already passing this flag.