Introduce loader for Meg V3 file format

[ocdi]

This code will parse a Meg file used by the CnC remastered assets. I used this code to successfully extract a music file and was able to play it locally.

#18239

[ocdi]

I got to the point where I was able to select this audio file from the list, however it didn't play because of unsupported audio format unfortunately. The compression type is ADPCM which I found some documentation here:

https://wiki.multimedia.cx/index.php/Microsoft_ADPCM

I see IMA_ADPCM is supported which seems simpler according to this

https://wiki.multimedia.cx/index.php/Microsoft_IMA_ADPCM

It looks like to finish this off, I'll have to add support to this audio format also.

[pchote]

I wouldn't rush on trying to get audio playback working in game: there are many pieces that need a lot more work before we will be able to ship that (reworking the filesystem/load code to support dynamic detection and use of the steam directory; reworking the music/notification systems to support split remaster vs classic definitions; adding the user-facing UI for selecting between the two). This PR should really focus on just the meg container format, and leave the other pieces to other PRs.

[ocdi]

@pchote thanks for reviewing while this was in draft. I mainly created it as such because I wanted to indicate to anyone else that there was a WIP that could be useful. I started going down the road of actually getting the audio to work because I thought that I was very close and didn't expect the blocker of unsupported wav file format. My main aim was to actually have the MegV3Loader exercised so that I could prove the code works.

Instead I have done what I should have and created two commands that can list and extract .meg files. I actually identified a bug with the SegmentStream where Read could read past the end of the of the indicated stream. In my extractor utility I was wondering why the .wav files were massive, presumably it was reading until the end of the underlying streams.

In practice I am unsure if that will affect anything, or break anything because it was incorrectly able to read past the end.

[pchote]

I haven't had a chance to test this yet, but here are some more code style points to hopefully restart progress here.

[pchote]

Confirmed that filesystem consumers can read the contents of mounted meg files without problem. I'll need to do another pass for coding style before approving, and the fixup commits will need to be squashed.

[ocdi]

I rebased on bleed and squashed

[pchote]

Sorry for the delay, you might have noticed that I got a bit sidetracked by what this PR enabled 🙂

A bunch more comments to bring this more in line with OpenRA's coding conventions, and a couple of specific simplifications:

[ocdi]

Thanks @pchote for your feedback. When I originally authored this we weren't targeting C# 7 so tuple's weren't an option; I am glad it is now! 👍 It is great to do the same stuff with less code.

[pchote]

A few final style nits, then LGTM.

These should bring us in line with the "proper" Megafile specs revealed in the new source release: https://github.com/electronicarts/CnC_Remastered_Collection/tree/master/CnCTDRAMapEditor/Utility

[ocdi]

I've updated the PR based on latest feedback and rebased on bleed.

[ocdi]

didn't notice the rebase label, have done so

[pchote]

LGTM after those last two nits.

[pchote]

Confirmed that this is still working fine on my updated remaster branch.

[abcdefg30]

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