Upgrade Mesa, to which version?

[devernay]

@rodlie says in #9 that we should maybe upgrade the Mesa version used by https://github.com/devernay/osmesa-install

But:

• Mesa dropped autoconf support with 19.0, supporting only meson and SCons builds. SCons support was also dropped in 21.1. Mangled OSMesa support was only accessible from autoconf and SCons, but it is just a matter of adding -DUSE_MGL_NAMESPACE to CPPFLAGS, so it should still be possible to easily get mangled OSMesa using meson.
• Source support for GL symbol mangling support was dropped with 19.2.2 (commit, relnotes). Re-adding it means we would have to maintain that large patch.
• The legacy swrast driver was dropped in Mesa 21.0.0.

Options are:

• Upgrade to 18.3.6 (DONE) and check if there are still thread-safety issues. I remember those issues were hard to spot, but maybe a Natron project with heavy multithreading and multiple shadertoy plugins doing software rendering would be a good test.
• Reapply the mangling patch to a more recent Mesa, which may require some work if functions were re-added to gl.h glext.h or glx.h since 18.3.6.

In the mean time:

• I upgraded OSMesa to 18.3.6, it still needs more testing in Natron.

Honestly, I think I'm fine with keeping 17.1.10, which is the safest option, since @rodlie already did the work of upgrading the scons buildsystem for that version (#9).

If 18.3.6 works too, we can upgrade to that version.

Please comment on that issue to give your thoughts.

[rodlie]

I'm ok with keeping 17.1.10.

supporting only meson and scons builds

scons has also been removed, meson is the only option in newer releases.

[devernay]

@rodlie I upgraded the version to 18.3.6, is it still building and running OK on mingw?

[rodlie]

Sorry for the late reply, been very busy this week. I will test after work today, will probably need to do some minor changes.