Moving the mGBA submodule upstream may be worthwhile

Draft is at https://github.com/mgba-emu/mgba/pull/3272. Original post follows.

Currently, we store our mGBA C code in https://github.com/TASVideos/mgba. I've been talking to @endrift lately, and it seems like there may be some benefits to moving this code to mGBA's upstream repository.

With mGBA being under active development, improvements and fixes are happening often and these are being carried over to BizHawk's port. However, we rely heavily on endrift to know which changes are safe enough to take downstream, and which ones we should wait on. Having the code in upstream would allow the safe commits to be directly applied by endrift and the unsafe ones skipped. This also would allow endrift more flexibility in redefining mGBA's API without leaving BizHawk behind.

This wouldn't mean we're taking a completely hands-off approach to mGBA. For one, we'd still have to manage the C# mGBA code, the dll and submodule in this repo as normal. We'd still be able to file PRs to mGBA repository as necessary. Currently, changes to mGBA are relatively infrequent on our end, compared to the amount of changes happening in upstream, so moving our code upstream could potentially increase overall efficiency.

Start by reading the linked mGBA draft pull request and comparing it with BizHawk's current C# mGBA code, DLL, and submodule arrangement. Confirm the ownership and integration changes discussed there; the work is done when the C code is maintained upstream while BizHawk's remaining integration continues to build and function normally.