Add IGUIScrollbar implementation with variable bar sizes (#8507)

src/gui/guiScrollBar.h

@@ -0,0 +1,71 @@
+/*
+Copyright (C) 2002-2013 Nikolaus Gebhardt
+This file is part of the "Irrlicht Engine".
+For conditions of distribution and use, see copyright notice in irrlicht.h
+
+Modified 2019.05.01 by stujones11, Stuart Jones <stujones111@gmail.com>
+
+This is a heavily modified copy of the Irrlicht CGUIScrollBar class
+which includes automatic scaling of the thumb slider and hiding of
+the arrow buttons where there is insufficient space.
+*/
+
+#pragma once
+
+#include "irrlichttypes_extrabloated.h"
+
+using namespace irr;
+using namespace gui;
+
+class guiScrollBar : public IGUIElement
+{
+public:
+	guiScrollBar(IGUIEnvironment *environment, IGUIElement *parent, s32 id,
+			core::rect<s32> rectangle, bool horizontal, bool auto_scale);
+
+	virtual void draw();
+	virtual void updateAbsolutePosition();
+	virtual bool OnEvent(const SEvent &event);
+	virtual void OnPostRender(u32 timeMs);
+
+	s32 getMax() const { return max_pos; }
+	s32 getMin() const { return min_pos; }
+	s32 getLargeStep() const { return large_step; }
+	s32 getSmallStep() const { return small_step; }
+	s32 getPos() const;
+
+	void setMax(const s32 &max);
+	void setMin(const s32 &min);
+	void setSmallStep(const s32 &step);
+	void setLargeStep(const s32 &step);
+	void setPos(const s32 &pos);
+	void setPageSize(const s32 &size);
+
+private:
+	void refreshControls();
+	s32 getPosFromMousePos(const core::position2di &p) const;
+	f32 range() const { return f32(max_pos - min_pos); }
+
+	IGUIButton *up_button;
+	IGUIButton *down_button;
+	bool is_dragging;
+	bool is_horizontal;
+	bool is_auto_scaling;
+	bool dragged_by_slider;
+	bool tray_clicked;
+	s32 scroll_pos;
+	s32 draw_center;
+	s32 thumb_size;
+	s32 min_pos;
+	s32 max_pos;
+	s32 small_step;
+	s32 large_step;
+	s32 desired_pos;
+	u32 last_change;
+	s32 drag_offset;
+	s32 page_size;
+	s32 border_size;
+
+	core::rect<s32> slider_rect;
+	video::SColor current_icon_color;
+};