Generate editor docs on a thread

[reduz]

• The main generation could not be moved to a thread, unfortunately, as it instantiates classes to get default values, interacts with ProjectSettings, etc.
• Only uncompressing documentation and merging it is threaded.
• Seems to improve editor load times by 0.5 seconds.

[Calinou]

Benchmark

❯ hyperfine -iw1 "bin/godot.linuxbsd.tools.64.llvm ~/Documents/Godot/test_decals_normal_orm_strength/project.godot --quit" "bin/godot.linuxbsd.tools.64.llvm.vanilla ~/Documents/Godot/test_decals_normal_orm_strength/project.godot --quit"

# This PR
Benchmark #1: bin/godot.linuxbsd.tools.64.llvm ~/Documents/Godot/test_decals_normal_orm_strength/project.godot --quit
  Time (mean ± σ):     10.020 s ±  0.363 s    [User: 8.625 s, System: 0.266 s]
  Range (min … max):    9.179 s … 10.439 s    10 runs

# `master` branch
Benchmark #2: bin/godot.linuxbsd.tools.64.llvm.vanilla ~/Documents/Godot/test_decals_normal_orm_strength/project.godot --quit
  Time (mean ± σ):     10.605 s ±  0.197 s    [User: 8.685 s, System: 0.261 s]
  Range (min … max):   10.243 s … 10.738 s    10 runs
 
Summary
  'bin/godot.linuxbsd.tools.64.llvm ~/Documents/Godot/test_decals_normal_orm_strength/project.godot --quit' ran
    1.06 ± 0.04 times faster than 'bin/godot.linuxbsd.tools.64.llvm.vanilla ~/Documents/Godot/test_decals_normal_orm_strength/project.godot --quit'

---

I backported this PR to 3.x and interestingly, it's slightly slower with the newer version:

❯ hyperfine -iw1 "bin/godot.x11.tools.64.llvm ~/Documents/Godot/escape-space/project.godot --quit" "bin/godot.x11.tools.64.llvm.vanilla ~/Documents/Godot/escape-space/project.godot --quit"

# This PR backported to `3.x`
Benchmark #1: bin/godot.x11.tools.64.llvm ~/Documents/Godot/escape-space/project.godot --quit
  Time (mean ± σ):      6.990 s ±  0.330 s    [User: 4.041 s, System: 0.200 s]
  Range (min … max):    6.302 s …  7.232 s    10 runs

# `3.x` branch
Benchmark #2: bin/godot.x11.tools.64.llvm.vanilla ~/Documents/Godot/escape-space/project.godot --quit
  Time (mean ± σ):      6.419 s ±  0.302 s    [User: 4.047 s, System: 0.194 s]
  Range (min … max):    5.832 s …  6.597 s    10 runs

Summary
  'bin/godot.x11.tools.64.llvm.vanilla ~/Documents/Godot/escape-space/project.godot --quit' ran
    1.09 ± 0.07 times faster than 'bin/godot.x11.tools.64.llvm ~/Documents/Godot/escape-space/project.godot --quit'

[akien-mga]

I don't see a direct use of Mutex in the added code.

[akien-mga]

Suggested change

	doc->generate(true); //not doable on threads, unfortunately since it instantiates all sorts of classes to get default values.
	doc->generate(true); // Not doable on threads unfortunately, since it instantiates all sorts of classes to get default values.

[akien-mga]

Commits should be squashed.

[akien-mga]

Tested after rebase against the current master branch, also seems to save 1.0s for me on a debug build:

$ hyperfine -iw1 "bin/godot.linuxbsd.tools.64.vanilla ~/tmp/godot/projects/FreeFall/project.godot --quit" "bin/godot.linuxbsd.tools.64.docthread ~/tmp/godot/projects/FreeFall/project.godot --quit"                                                                           
Benchmark #1: bin/godot.linuxbsd.tools.64.vanilla ~/tmp/godot/projects/FreeFall/project.godot --quit
  Time (mean ± σ):      9.148 s ±  0.334 s    [User: 7.238 s, System: 0.327 s]
  Range (min … max):    8.545 s …  9.398 s    10 runs
 
  Warning: Ignoring non-zero exit code.
  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet PC without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.
 
Benchmark #2: bin/godot.linuxbsd.tools.64.docthread ~/tmp/godot/projects/FreeFall/project.godot --quit
  Time (mean ± σ):      8.362 s ±  0.402 s    [User: 7.261 s, System: 0.341 s]
  Range (min … max):    7.845 s …  8.917 s    10 runs
 
  Warning: Ignoring non-zero exit code.
 
Summary
  'bin/godot.linuxbsd.tools.64.docthread ~/tmp/godot/projects/FreeFall/project.godot --quit' ran
    1.09 ± 0.07 times faster than 'bin/godot.linuxbsd.tools.64.vanilla ~/tmp/godot/projects/FreeFall/project.godot --quit'

Changes done.

[akien-mga]

Thanks!