virtualcam-module: Make placeholder initialization thread-safe

[notr1ch]

Description

A single application can create any number of VCamFilter instances, all of which create a thread that tries to initialize the placeholder image. This can result in a race where the placeholder vector gets resized by multiple threads, causing dangling pointers and a crash.

This PR makes placeholder initialization thread-safe by using C++11 function-local static initialization, ensuring only one thread runs the initialization and the rest wait for it. State was moved to a struct to ensure it can be updated and passed around atomically. A missing return check from GdiplusStartup was also added.

Note: this goes against a CODESTYLE.md section that says:

Use thread-safe functions to initialize a function-local static variable

Initialization of function-local static variables is inherently thread-safe in C++11 and later, so adding additional synchronization seemed unnecessary.

Motivation and Context

As the virtual camera gets injected into any application enumerating Directshow devices, we should be a good citizen and not crash the host application.

How Has This Been Tested?

Loaded the new filter and tested that the placeholder appeared. The actual crash I wasn't able to reproduce given it's a race.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested (kind of).
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Does GdiplusShutdown() only need to be called if GdiplusStartup() succeeds? If it needs to be called all the time, this could instead be something like:

	Placeholder result;
	Status s = GdiplusStartup(&token, &si, nullptr);
	if (s != Status::Ok) {
		result = Placeholder();
	} else {
		result = load_placeholder_internal();
	}

	GdiplusShutdown(token);

[notr1ch]

The documentation isn't specific, but I imagine a failure means the token isn't returned, so it's undefined behavior to pass it to GdiplusShutdown.