raytracer: use lighter triangle class to reduce memory consumption

[adrientoub]

• should speed up all memory copies
• should improve data locality
• should reduce memory consumption (a lot) of meshes
• removes duplicate attributes & color copied in mesh
• breaking change, parsing of Mesh is changed
  • old:
    mesh name
    triangle name a b c attr color
    ...
    end
  • new
    mesh name attr color
    triangle a b c
    ...
    end
    triangle names had no use at all and attributes and color were
    duplicates across all lines

[adrientoub]

Before:
==12902== HEAP SUMMARY:
==12902== in use at exit: 0 bytes in 0 blocks
==12902== total heap usage: 29,302,973 allocs, 29,302,973 frees, 1,408,478,583 bytes allocated

After:
==12744== HEAP SUMMARY:
==12744== in use at exit: 0 bytes in 0 blocks
==12744== total heap usage: 29,227,774 allocs, 29,227,774 frees, 1,402,713,421 bytes allocated