[PCKPacker] Add method to add files from buffer.

[bruvzg]

Fixes godotengine/godot-proposals#12806
Also fixes off by one file index in the verbose flush print.

It was problematic before 4.4, since it was storing all files at once at flush, but in 4.5 files are saved immediately, so it makes sense to allow writing from buffer.

[Calinou]

Tested locally, it works as expected.

Code looks good to me.

This unit test can be added after tests/test_pck_packer.h:102:

CHECK_MESSAGE(
			pck_packer.add_file_from_buffer("buffer/new.txt", String("Hello world!").to_utf8_buffer()) == OK,
			"Adding a file from a buffer to the PCK in a new subdirectory should return an OK error code.");

[bruvzg]

This unit test can be added after tests/test_pck_packer.h:102

Done.

[sinni800]

Many thanks!

EDIT: I'm currently using this productively with great success. Will this still make it into 4.5? Otherwise I'll just keep adding this as a patch onto my own build,

EDIT: I've overlooked that it is 4.6, woops.

[sinni800]

Can this be merged? It's a very low risk change that doesn't really impact anything negatively, anyway.

[Calinou]

I meant to approve this after the unit test was added, and it's here now.

[sinni800]

thank you :)

[Repiteo]

Thanks!