Update Mesa to v26.0.3 with softpipe for all non-macOS platforms

[ViralBShah]

Summary

• Update Mesa from v20.1.5 to v26.0.3 (latest stable)
• Add all Linux, FreeBSD, and Windows platforms (exclude macOS which provides OpenGL natively)
• Use softpipe software renderer (no LLVM dependency to avoid conflicts with Julia's LLVM)
• Require GCC 10+ for C++17 support (Mesa 26 requirement)
• Add X11 dependencies for Linux/FreeBSD (libX11, libXext, libxcb, xorgproto, libxshmfence, libXrandr, libXxf86vm)

Addresses #3012

Test plan

• Built successfully for x86_64-linux-gnu — produces libGL.so.1.5.0
• Built successfully for x86_64-w64-mingw32 — produces opengl32sw.dll
• Built successfully for i686-w64-mingw32 — produces opengl32sw.dll
• CI build for all platforms

🤖 Generated with Claude Code

[ViralBShah]

@giordano Is the use of LLVM correct in here? The package uses LLVM internally to speed up the rendering pipeline - and does not (should not) interact with Julia's LLVM.

[giordano]

I'm not sure. Statically linking llvm means that this library would bring yet another llvm in memory, which is potentially dangerous. Comments seem to suggest only llvmpipe is used? I don't know what that is though.

[ViralBShah]

Ok let me see if I can make it link dynamically. This package is not really used anywhere for the moment, so there won't be a real issue anywhere.

[ViralBShah]

I decided it isn't worth it to build with LLVM for now - since most users will have a real GPU anyways. Let's see if this package ends up being useful, and if necessary we can enable LLVM in the future.