Create a "GameplayHelpers" file and move the margin gathering code

in there.

src/GameplayHelpers.cpp

@@ -0,0 +1,57 @@
+#include <vector>
+
+#include "global.h"
+
+#include "EnumHelper.h"
+#include "GameplayHelpers.h"
+#include "GameState.h"
+#include "LuaManager.h"
+#include "ThemeManager.h"
+#include "Style.h"
+
+std::vector<NotefieldMargins> GetNotefieldMargins() {
+	LuaReference margin_func;
+	std::vector<NotefieldMargins> margins(2);
+
+	THEME->GetMetric("ScreenGameplay", "MarginFunction", margin_func);
+	if (margin_func.GetLuaType() != LUA_TFUNCTION)
+	{
+		LuaHelpers::ReportScriptErrorFmt("MarginFunction metric for %s must be a function.", "ScreenGameplay");
+		return margins;
+	}
+
+	// Setup the lua.
+	Lua* lua_ptr = LUA->Get();
+	margin_func.PushSelf(lua_ptr);
+	lua_createtable(lua_ptr, 0, 0);
+	int next_player_slot = 1;
+	FOREACH_EnabledPlayer(pn)
+	{
+		Enum::Push(lua_ptr, pn);
+		lua_rawseti(lua_ptr, -2, next_player_slot);
+		++next_player_slot;
+	}
+
+	Enum::Push(lua_ptr, GAMESTATE->GetCurrentStyle(PLAYER_INVALID)->m_StyleType);
+	RString err = "Error running MarginFunction: ";
+
+	// Run the lua code.
+	if (LuaHelpers::RunScriptOnStack(lua_ptr, /*Error=*/err, /*Args=*/2, /*ReturnValues*/3, /*ReportOnError*/true))
+	{
+		std::string margin_error_msg = "Margin value must be a number.";
+
+		// Pull the values off the lua stack. Note that the lua function is
+		// returning the margins of P1 + P2 combined. Therefore, we need to use
+		// the center to tell where P1's margin ends and P2's begins, if
+		// applicable.
+		margins[PLAYER_1].left = SafeFArg(lua_ptr, -3, margin_error_msg, 40);
+		float center = SafeFArg(lua_ptr, -2, margin_error_msg, 80);
+		margins[PLAYER_1].right = center / 2.0f;
+
+		margins[PLAYER_2].left = center / 2.0f;
+		margins[PLAYER_2].right = SafeFArg(lua_ptr, -1, margin_error_msg, 40);
+	}
+	lua_settop(lua_ptr, 0);
+	LUA->Release(lua_ptr);
+	return margins;
+}