Add Optional Inclusion of libwebp in Third Party Libaries

[benn-herrera]

Add libwebp to third_party

This PR broken out as 2 of 3 from #9363 per request by @pixelflinger

• Addresses Issue 7962
• Added third_party/libwebp at version 1.6.0
  • Vast majority of the file changes
• Added integration in third_party/libwebp/tnt/
  • update_libwebp.sh (modified version of third_party/cgltf/tnt/update_cgltf.sh from Update CGLTF to get Webp Extension Support #9369
  • CMakeLists.txt that sets build options and wraps main libwebp project cmake
• Added FILAMENT_SUPPORTS_WEBP_TEXTURES cmake build option
  • controls inclusion of third_party/libwebp/tnt
  • adds compile time macro definition FILAMENT_SUPPORTS_WEBP_TEXTURES when ON
  • defaults to OFF

[z3moon]

I worry that this might force the application that consumes this library (Filament in this case) to always enable multi-thread support.

Some of our WebAssembly users (wasm) may want to disable pthread for their product. In this case, I wonder how this option plays out.

[benn-herrera]

that's a good point. for web assembly builds SIMD shoiuld be off as should thread support.

[z3moon]

I'm not sure how WEBGL_PTHREADS is supposed to be set for Filament. But it's outside the scope of this pr.

[benn-herrera]

Hey, @pixelflinger - I have a branch on my fork ready for opening the last PR adding optional webp texture support here once this PR and #9369 are merged.