mame: use biggest int multiple of 60 as a framerate numerator

denominator is determined by taking refresh attoseconts `as_ticks(numerator)` which reduces the initial 64bit value to what can be used for video

mame needs flat 60fps for screenless machines, so we use a numerator that is a multiple, while also being a tiny bit more accurate than with 1,000,000,000

Author: vadosnaprimer

src/BizHawk.Emulation.Cores/Arcades/MAME/MAME.IVideoProvider.cs

@@ -27,9 +27,9 @@ public partial class MAME : IVideoProvider
 
 		private void UpdateFramerate()
 		{
-			VsyncNumerator = 1000000000;
 			VsyncAttoseconds = _core.mame_lua_get_long(MAMELuaCommand.GetRefresh);
-			VsyncDenominator = (int)(VsyncAttoseconds / 1000000000);
+			VsyncNumerator = 0x7ffffff8;
+			VsyncDenominator = _core.mame_lua_get_int(MAMELuaCommand.GetFramerateDenominator(VsyncNumerator));
 		}
 
 		private void UpdateAspect()

src/BizHawk.Emulation.Cores/Arcades/MAME/MAME.cs

@@ -331,6 +331,10 @@ private static class MAMELuaCommand
 				"table.sort(final) " +
 				"return table.concat(final)";
 
+			public static string GetFramerateDenominator(int frequency) =>
+				"for k,v in pairs(manager.machine.screens) do " +
+					$"return emu.attotime(0, v.refresh_attoseconds):as_ticks({ frequency }) " +
+				"end";
 			public static string MakeLookupKey(string gameName, string luaCode) =>
 				$"[{ gameName }] { luaCode }";
 			public static string InputField(string tag, string fieldName) =>