Preferences: Implement a font scaling option

This is currently incomplete - it only works for GUI2 text.

data/gui/default/window/preferences/02_display.cfg

@@ -208,6 +208,21 @@
 			[/grid]
 		[/column]
 	[/row]
+
+	[row]
+		[column]
+			horizontal_grow = true
+			{_GUI_PREFERENCES_CHECKBOX_ALIGN_BORDER}
+			{_GUI_PREFERENCES_MAIN_COMPOSITE_SLIDER
+				scaling_value ( _ "Percentage Font Scaling:")
+				scaling_slider (
+					minimum_value,maximum_value=100,200
+					step_size=5
+					tooltip= _ "Set the scaling factor of fonts"
+				)
+			}
+		[/column]
+	[/row]
 #enddef
 
 #define _GUI_PREFERENCES_DISPLAY_GRID_2

src/gui/dialogs/preferences_dialog.cpp

@@ -554,6 +554,12 @@ void tpreferences::initialize_members(twindow& window)
 		idle_anim(), idle_anim_rate(),
 		set_idle_anim, set_idle_anim_rate, window);
 
+	/** FONT SCALING **/
+	tslider& scale_slider = find_widget<tslider>(&window, "scaling_slider", false);
+	scale_slider.set_value(font_scaling()); // This shouldn't be necessary, but it won't work without it...
+	register_integer("scaling_slider", false, font_scaling, set_font_scaling);
+	bind_status_label(scale_slider, "scaling_value", window);
+
 	/** SELECT THEME **/
 	connect_signal_mouse_left_click(
 			find_widget<tbutton>(&window, "choose_theme", false),

src/preferences.cpp

@@ -411,6 +411,22 @@ void save_turbo_speed(const double speed)
 {
 	prefs["turbo_speed"] = speed;
 }
+
+int font_scaling()
+{
+	// Clip at 50 because if it's too low it'll cause crashes
+	return std::max<int>(50, prefs["font_scale"].to_int(100));
+}
+
+void set_font_scaling(int scale)
+{
+	prefs["font_scale"] = scale;
+}
+
+int font_scaled(int size)
+{
+	return (size * font_scaling()) / 100;
+}
 
 bool idle_anim()
 {

src/preferences.hpp

@@ -81,6 +81,10 @@ namespace preferences {
 
 	double turbo_speed();
 	void save_turbo_speed(const double speed);
+
+	int font_scaling();
+	void set_font_scaling(int scale);
+	int font_scaled(int size);
 
 	bool idle_anim();
 	void _set_idle_anim(const bool ison);

src/text.cpp

@@ -24,6 +24,7 @@
 #include "serialization/string_utils.hpp"
 #include "serialization/unicode.hpp"
 #include "tstring.hpp"
+#include "preferences.hpp"
 
 #include <boost/foreach.hpp>
 
@@ -404,8 +405,9 @@ ttext& ttext::set_family_class(font::family_class fclass)
 
 ttext& ttext::set_font_size(const unsigned font_size)
 {
-	if(font_size != font_size_) {
-		font_size_ = font_size;
+	unsigned int actual_size = preferences::font_scaled(font_size);
+	if(actual_size != font_size_) {
+		font_size_ = actual_size;
 		calculation_dirty_ = true;
 		surface_dirty_ = true;
 	}