Implement an editor to customize the touchscreen controls (#14933)

- The editor is accessible via the pause menu and the settings menu.
- Buttons can be moved via drag & drop.
- Buttons can be added/removed. The grid menu added by #14918 is used to show
  all buttons not included in the layout.
- Custom layouts are responsive and adapt to changed screen size / DPI /
  hud_scaling.
- The layout is saved as JSON in the "touch_layout" setting.

src/gui/touchscreenlayout.h

@@ -0,0 +1,104 @@
+// Luanti
+// SPDX-License-Identifier: LGPL-2.1-or-later
+// Copyright (C) 2024 grorp, Gregor Parzefall <grorp@posteo.de>
+
+#pragma once
+
+#include "irr_ptr.h"
+#include "irrlichttypes_bloated.h"
+#include "rect.h"
+#include <iostream>
+#include <unordered_map>
+
+class ISimpleTextureSource;
+namespace irr::gui
+{
+	class IGUIStaticText;
+}
+namespace irr::video
+{
+	class ITexture;
+}
+
+enum touch_gui_button_id : u8
+{
+	jump_id = 0,
+	sneak_id,
+	zoom_id,
+	aux1_id,
+	overflow_id,
+
+	// formerly "rare controls bar"
+	chat_id,
+	inventory_id,
+	drop_id,
+	exit_id,
+
+	// formerly "settings bar"
+	fly_id,
+	fast_id,
+	noclip_id,
+	debug_id,
+	camera_id,
+	range_id,
+	minimap_id,
+	toggle_chat_id,
+
+	// the joystick
+	joystick_off_id,
+	joystick_bg_id,
+	joystick_center_id,
+
+	touch_gui_button_id_END,
+};
+
+extern const char *button_names[];
+extern const char *button_titles[];
+extern const char *button_image_names[];
+
+struct ButtonMeta {
+	// Position, specified as a percentage of the screensize in the range [0,1].
+	// The editor currently writes the values 0, 0.5 and 1.
+	v2f position;
+	// Offset, multiplied by the global button size before it is applied.
+	// Together, position and offset define the position of the button's center.
+	v2f offset;
+
+	// Returns the button's effective center position in pixels.
+	v2s32 getPos(v2u32 screensize, s32 button_size) const;
+	// Sets the button's effective center position in pixels.
+	void setPos(v2s32 pos, v2u32 screensize, s32 button_size);
+};
+
+struct ButtonLayout {
+	static bool isButtonAllowed(touch_gui_button_id id);
+	static bool isButtonRequired(touch_gui_button_id id);
+	static s32 getButtonSize(v2u32 screensize);
+	static ButtonLayout loadFromSettings();
+
+	static video::ITexture *getTexture(touch_gui_button_id btn, ISimpleTextureSource *tsrc);
+	static void clearTextureCache();
+
+	std::unordered_map<touch_gui_button_id, ButtonMeta> layout;
+
+	core::recti getRect(touch_gui_button_id btn,
+			v2u32 screensize, s32 button_size, ISimpleTextureSource *tsrc);
+
+	std::vector<touch_gui_button_id> getMissingButtons();
+
+	void serializeJson(std::ostream &os) const;
+	void deserializeJson(std::istream &is);
+
+	static const ButtonLayout predefined;
+
+private:
+	static std::unordered_map<touch_gui_button_id, irr_ptr<video::ITexture>> texture_cache;
+};
+
+void layout_button_grid(v2u32 screensize, ISimpleTextureSource *tsrc,
+		const std::vector<touch_gui_button_id> &buttons,
+		// pos refers to the center of the button.
+		const std::function<void(touch_gui_button_id btn, v2s32 pos, core::recti rect)> &callback);
+
+void make_button_grid_title(gui::IGUIStaticText *text,
+		touch_gui_button_id btn,v2s32 pos, core::recti rect);