contrib: package glad2, libplacebo5

I'm submitting this as a draft because of the resulting open points:

• Is this approach of building python dependencies inside a virtualenv agreeable?
• Right now, as written, this pip install $< command results in pip automatically pulling in further sub-dependencies of glad2 via the internet (currently only jinja2, and it own sub-dependent MarkupSafe). Should we also add MarkupSafe and jinja2 to our contribs via the same mechanism?
• Needs to be updated to drop the -rc2 suffix when libplacebo-v5.228.0 is formally released