DS - add a DS lua library with a touchScreenStart() method, add a cross hair stylus input display script

Assets/Lua/DS/StyleusInputDisplay.lua

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+-- Gives a cross hair UI for the stylus for DS games
+
+local upColor = 'white'
+local downColor = 'green'
+local dotColor = 'red'
+
+function Draw(x, y, maxX, maxY, isDown)
+	color = upColor
+	if isDown then
+		color = downColor
+	end
+
+	gui.drawLine(0, y, maxX, y, color)
+	gui.drawLine(x, 0, x, maxY, color)
+
+	if isDown then
+		gui.drawPixel(x, y, dotColor)
+	end
+end
+
+while true do
+	if emu.getsystemid() ~= "NDS" then
+		console.log('This script is for Nintendo DS only')
+		break
+	end
+	
+	local touchStart = ds.touchScreenStart()
+	if touchStart then
+		local bufferX = client.bufferwidth()
+		local bufferY = client.bufferheight()
+
+		local btns = joypad.get()
+		local x = btns['TouchX']
+		local y = btns['TouchY']
+		local isDown = btns['Touch']
+
+		-- A bit of a hack to ensure it is not drawing while mouse moving
+		-- on the top screen
+		if y == 0 then
+			x = 0
+		end
+
+		x = x + touchStart.X
+		y = y + touchStart.Y
+
+		Draw(x, y, bufferX, bufferY, isDown)
+	end
+
+	emu.frameadvance()
+end

BizHawk.Client.Common/lua/EmuLuaLibrary.DS.cs

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+using System;
+using System.ComponentModel;
+using System.Drawing;
+using BizHawk.Emulation.Common;
+using BizHawk.Emulation.Cores.Consoles.Nintendo.NDS;
+using NLua;
+
+// ReSharper disable UnusedMember.Global
+namespace BizHawk.Client.Common.lua
+{
+	[Description("Functions specific to DSHawk (functions may not run when an Genesis game is not loaded)")]
+	public sealed class DSLuaLibrary : DelegatingLuaLibrary
+	{
+		public DSLuaLibrary(Lua lua)
+			: base(lua) { }
+
+		public DSLuaLibrary(Lua lua, Action<string> logOutputCallback)
+			: base(lua, logOutputCallback) { }
+
+		public override string Name => "ds";
+
+		[RequiredService]
+		public IEmulator Emulator { get; set; }
+
+		[LuaMethodExample("touchStartX = ds.touchScreenStart().X")]
+		[LuaMethod("touchScreenStart", "Gets the buffer coordinates that represent the start of the touch screen area. If the touch screen is not currently being displayed, nil will be returned.")]
+		public Point? TouchScreenStart()
+		{
+			if (Emulator is MelonDS ds)
+			{
+				return ds.TouchScreenStart;
+			}
+
+			return null;
+		}
+	}
+}