ci: add a script to extract the OSMesa build

This will allow the CI to extract a useful image on top of the latest
`manylinux2014` image instead of sticking with whatever the image
happened to be built with.

.gitlab/ci/docker/python-osmesa/README.md

+## OSMesa wheel docker
+
+This Docker image builds LLVM and OSMesa to facilitate making VTK wheels which
+use OSMesa instead of the system OpenGL for rendering. Note that Docker images
+are not friendly to "layering" shifts and while this does make *an* image, it
+is not used in CI. Instead, the `extract.sh` script is used to pull the built
+OSMesa from the image and used to make a tarball that is extracted at CI time.
+This allows the Python base image to be updated with new Python releases
+without having to rebuild our OSMesa all the time.

.gitlab/ci/docker/python-osmesa/extract.sh

+#!/bin/sh
+
+set -e
+
+if command -v podman >/dev/null 2>/dev/null; then
+    docker="podman"
+else
+    docker="docker"
+fi
+readonly docker
+
+readonly image="$1"
+shift
+
+arch="$( uname -m )"
+readonly arch
+
+version="$( grep mesa_version= install_osmesa.sh | cut -d\" -f2 )"
+readonly version
+
+date="$( date +%Y%m%d )"
+readonly date
+
+exec $docker run --rm "$image" tar cJf - /opt/osmesa > "vtk-osmesa-$version-$arch-$date.tar.xz"