Implement proper serialization of OptionalContentConfig

[Snuffleupagus]

I happened to notice that the way the OptionalContentConfig-data handled in the PR that implements worker-rendering leaves a lot to be desired:

• The way that the optional content state is handled is not correct, since that PR collects the "effective visibility" of the optional content groups rather than their actual internal state.

• The necessary OptionalContentConfig-data is collected piecemeal in the API, which leads to quite frankly very messy code that's hard to read and will be even harder to maintain.

The solution to all of these issues seem really simple though, just add a couple of OptionalContentConfig methods that serialize/de-serialize the necessary data.
In the API calling optionalContentConfig.serializable will get all of the needed data for transferring to the worker-renderer, and once received there calling OptionalContentConfig.fromSerializable(/* transferred data here */) will create an OptionalContentConfig instance with the correct internal state.

As part of this patch, to avoid increasing bundle-size unnecessarily, a couple of existing methods are stubbed out when the OptionalContentConfig class ends up in a worker-file (since they're unused there).
This part assumes that the new worker-renderer is built correctly, note how the existing pdf.worker.mjs is handled in

pdf.js/gulpfile.mjs

Lines 539 to 545 in 03eda70

	function createWorkerBundle(defines) {
	const workerDefines = {
	...defines,
	WORKER_THREAD: true,
	};
	const workerFileConfig = createWebpackConfig(workerDefines, {
	filename: workerDefines.MINIFIED ? "pdf.worker.min.mjs" : "pdf.worker.mjs",

(Note: Submitting a PR was a lot faster than trying to provide review comments, since writing this commit message took longer than writing the patch.)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 38.46154% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.41%. Comparing base (b849567) to head (ce45d5a).

Files with missing lines	Patch %	Lines
src/display/optional_content_config.js	38.46%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21349      +/-   ##
==========================================
- Coverage   81.42%   81.41%   -0.01%     
==========================================
  Files         257      257              
  Lines       65351    65367      +16     
==========================================
+ Hits        53212    53219       +7     
- Misses      12139    12148       +9     

Flag	Coverage Δ
fonttest	9.10% <ø> (ø)
integrationtest	67.00% <38.46%> (+<0.01%)	⬆️
unittest	56.85% <34.61%> (-0.03%)	⬇️
unittestcli	56.06% <34.61%> (-0.02%)	⬇️

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[Snuffleupagus]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/a730ab861be62a6/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.193.163.58:8877/f91ab7b6aa5a48f/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Failed

Full output at http://54.241.84.105:8877/a730ab861be62a6/output.txt

Total script time: 19.40 mins

• Regression tests: FAILED

  errors: 5

Image differences available at: http://54.241.84.105:8877/a730ab861be62a6/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/f91ab7b6aa5a48f/output.txt

Total script time: 25.43 mins

• Regression tests: Passed

[timvandermeij]

Looks good to me; thanks!