Add obj importer changes to use ImporterMesh.

[fire]

Fixes: godotengine/godot-proposals#8600

Note: this switches the ImporterMesh round tripping to be on the common track and not on the niche track.

[fire]

Instead of converting from ImporterMesh to ArrayMesh using get_mesh(). In my next revision I will use a boolean that defaults to off to swtich between ImporterMesh and ArrayMesh

[adamscott]

Fine by me! I tested it and it works. Thanks @fire!

[reduz]

Looks perfect!

[akien-mga]

Thanks!

[Calinou]

When importing the same OBJ file 10 times, this seems to be slightly slower when comparing optimized builds together between master and this PR. This PR also doesn't resolve #84357.

The difference isn't dramatic so this is probably within acceptable bounds, especially if it helps resolve bugs and make code maintenance easier in the future.

Testing project: test_obj_import.zip (make 10 copies of octavus.obj within the project folder)

❯ hyperfine -iw1 -p "rm -rf ~/Documents/Godot/test_obj_import/.godot" "bin/godot.linuxbsd.editor.x86_64.master ~/Documents/Godot/test_obj_import/project.godot --quit-after 5" "bin/godot.linuxbsd.editor.x86_64 ~/Documents/Godot/test_obj_import/project.godot --quit-after 5"

Benchmark 1: bin/godot.linuxbsd.editor.x86_64.master ~/Documents/Godot/test_obj_import/project.godot --quit-after 5
  Time (mean ± σ):      7.464 s ±  0.233 s    [User: 6.866 s, System: 0.542 s]
  Range (min … max):    7.324 s …  7.905 s    10 runs

Benchmark 2: bin/godot.linuxbsd.editor.x86_64 ~/Documents/Godot/test_obj_import/project.godot --quit-after 5
  Time (mean ± σ):      7.648 s ±  0.342 s    [User: 6.874 s, System: 0.551 s]
  Range (min … max):    6.891 s …  7.943 s    10 runs
 
Summary
  bin/godot.linuxbsd.editor.x86_64.master ~/Documents/Godot/test_obj_import/project.godot --quit-after 5 ran
    1.02 ± 0.06 times faster than bin/godot.linuxbsd.editor.x86_64 ~/Documents/Godot/test_obj_import/project.godot --quit-after

[akien-mga]

Cherry-picked for 4.2.2.