libsnes: Cache region from core once at start

The region value in the comm struct is set once and then gets wiped out later.  I don't know what wipes it, but so many things have their hands on that, it's not surprising.  Someone knew about this and handled _mapper appropriately, but not _region.

Fixes #2503

src/BizHawk.Emulation.Cores/Consoles/Nintendo/SNES/LibsnesCore.IMemoryDomains.cs

@@ -11,6 +11,7 @@ public partial class LibsnesCore
 		private readonly List<MemoryDomain> _memoryDomainList = new List<MemoryDomain>();
 		private IMemoryDomains _memoryDomains;
 		private LibsnesApi.SNES_MAPPER? _mapper;
+		private LibsnesApi.SNES_REGION? _region;
 
 		// works for WRAM, garbage for anything else
 		private static int? FakeBusMap(int addr)

src/BizHawk.Emulation.Cores/Consoles/Nintendo/SNES/LibsnesCore.IRegionable.cs

@@ -4,7 +4,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.SNES
 {
 	public partial class LibsnesCore : IRegionable
 	{
-		public DisplayType Region => Api.Region == LibsnesApi.SNES_REGION.NTSC
+		public DisplayType Region => _region == LibsnesApi.SNES_REGION.NTSC
 			? DisplayType.NTSC
 			: DisplayType.PAL;
 	}

src/BizHawk.Emulation.Cores/Consoles/Nintendo/SNES/LibsnesCore.cs

@@ -173,7 +173,7 @@ public unsafe partial class LibsnesCore : IEmulator, IVideoProvider, ISaveRam, I
 				}
 			}
 
-			if (Api.Region == LibsnesApi.SNES_REGION.NTSC)
+			if (_region == LibsnesApi.SNES_REGION.NTSC)
 			{
 				// similar to what aviout reports from snes9x and seems logical from bsnes first principles. bsnes uses that numerator (ntsc master clockrate) for sure.
 				VsyncNumerator = 21477272;
@@ -484,6 +484,7 @@ private bool LoadCurrent()
 				: Api.CMD_load_cartridge_super_game_boy(_currLoadParams.rom_xml, _currLoadParams.rom_data, _currLoadParams.rom_size, _currLoadParams.dmg_data);
 
 			_mapper = Api.Mapper;
+			_region = Api.Region;
 
 			return result;
 		}