Converting mesa/demos to Meson

[Erik Faye-Lund]

Because we've landed on using Meson in the main Mesa repository, I've
been working on converting the mesa/demos repo to Meson as well.

I've posted an MR here:
https://gitlab.freedesktop.org/mesa/demos/-/merge_requests/60

This MR adds a new Meson build system to the repository, and marks the
Autotools and CMake build systems as deprecated, similar to what we did
for the Autotools and SCons build systems in Mesa when we transitioned.

After this lands, I propose that we cut a new release (we really should
cut a new release soon anyway), then wait for a while and fix up any
problems found, and finally rip ot the old build systems.

After removing the old build systems, we end up with this total code
reduction:

112 files changed, 1642 insertions(+), 4744 deletions(-)

The removal commit is here, BTW:
https://gitlab.freedesktop.org/kusma/mesa-demos/-/commits/remove-old-buildsystems

Thoughts? Objections?

-- 
Erik Faye-Lund
Principal Engineer

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