VideoCommon: Move backend_info out of VideoConfig struct.

[jordan-woyak]

I plan to clean up the way VideoConfig is handled and this is step 1.

Both g_Config and g_ActiveConfig had their own copy of backend_info. This was unnecessary since the members were just set once upon VideoBackend initialization.

I've moved backend_info from the VideoConfig struct to be its own global, BackendInfo g_backend_info. This still isn't ideal, and it should probably exist inside each VideoBackend, but it's a baby step in the right direction and I didn't want to make the changes too drastic in one PR.

This unfortunately changes a lot of files but I'll explain what I've done:

• Replaced all occurrences of g_ActiveConfig.backend_info with g_backend_info.
• Replaced all occurrences of g_Config.backend_info with g_backend_info.
• Adjusted a few Vulkan/Metal functions that populated a VideoConfig to instead populate a BackendInfo.
• Assessed that all assignments to BackendInfo members occur specifically from VideoBackend initialization.
  • The only exception that I see is bSupportsExclusiveFullscreen. Vulkan on Windows can change this during emulation, but it assigned the value in both g_Config and g_ActiveConfig simultaneously so it was still unnecessary to have two copies. I'll mention that the way this value is used across threads was pretty hacky and it still is, but I'm saving that for another day.

I see now that this is redoing work of #11531. That PR certainly has a cleaner end result, but it changes a lot and is in need of a rebase. I'm hoping we can get things moving with smaller steps like I've done here.

[BhaaLseN]

Pretty straight-forward change, looks good (but untested).

[iwubcode]

If we're going to wait on that one change. LGTM.

[jordan-woyak]

If we're going to wait on that one change. LGTM.

Alright, fine, I changed the one thing. :)

[Dentomologist]

LGTM, slightly tested by starting up a game with a few backends.