Add a software-only libGL in the casa-run-5.3 image

[ylep]

We used to ship a software-only libGL in casa-run-5.0 but it was dropped during the upgrade to newer Ubuntu versions. Here it is back for the casa-run-5.3 image, but this time it is compiled during the creation of the image instead of being shipped pre-compiled (it is actually quite fast to compile).

This is part of an attempt to fix #321 but a software-only libGL could be useful in other scenarios.

I have built the new images and am ready to push them if you approve this PR :-)

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[ylep]

If you don't mind, I would prefer to wait until we better understand #321 before actually adding the software-only libGL: we still don't know if it is really required, or maybe the system-provided mesa can work with the LIBGL_ALWAYS_SOFTWARE environment variable? (see https://docs.mesa3d.org/envvars.html#envvar-LIBGL_ALWAYS_SOFTWARE)

[denisri]

So I let you test further using the LIBGL_ALWAYS_SOFTWARE env variable ?

[ylep]

LIBGL_ALWAYS_SOFTWARE does not ameliorate any of the issues described in #321.

As a result, I propose to include the software-only libGL in the image even though it does not help with older X2Go (4.0.0), because it still allows to fix glxinfo and glxgears (again, see #321).