Cache measurement of space in TextStyle

I noticed a ton of string_length() calls on empty space strings,
each one making an sf::Text object to measure. I thought caching
the result would make the slow dialogs open faster but it didn't.
Still might as well do it, though.

src/gfx/render_text.cpp

@@ -9,12 +9,13 @@
 #include "render_text.hpp"
 
 #include <iostream>
+#include <set>
 #include "fileio/resmgr/res_font.hpp"
 #include "gfx/render_shapes.hpp"
 #include <utility>
 #include "winutil.hpp"
 
-void TextStyle::applyTo(sf::Text& text, double scale) {
+void TextStyle::applyTo(sf::Text& text, double scale) const {
 	switch(font) {
 		case FONT_PLAIN:
 			text.setFont(*ResMgr::fonts.get("plain"));
@@ -319,13 +320,26 @@ std::vector<snippet_t> draw_string_sel(sf::RenderTarget& dest_window,rectangle d
 	return params.snippets;
 }
 
-size_t string_length(std::string str, TextStyle style, short* height){
+std::set<std::string> strings_to_cache = {" "};
+
+size_t string_length(std::string str, const TextStyle& style, short* height){
 	size_t total_width = 0;
+	if(style.measurementCache.find(str) != style.measurementCache.end()){
+		location measurement = style.measurementCache[str];
+		if(height) *height = measurement.y;
+		return measurement.x;
+	}
 	
 	sf::Text text;
 	style.applyTo(text);
 	text.setString(str);
 	total_width = text.getLocalBounds().width;
+	if(strings_to_cache.count(str)){
+		location measurement;
+		measurement.x = total_width;
+		measurement.y = text.getLocalBounds().height;
+		style.measurementCache[str] = measurement;
+	}
 	if(height) *height = text.getLocalBounds().height;
 	return total_width;
 }

src/gfx/render_text.hpp

@@ -36,7 +36,8 @@ struct TextStyle {
 	int pointSize = 10, lineHeight = 10;
 	sf::Color colour;
 	TextStyle() : colour(sf::Color::Black) {}
-	void applyTo(sf::Text& text, double scale = 1.0);
+	void applyTo(sf::Text& text, double scale = 1.0) const;
+	mutable std::map<std::string, location> measurementCache;
 };
 
 // elements: std::make_tuple(last_line_break, last_word_break, line_width)
@@ -63,6 +64,6 @@ void clear_scale_aware_text(sf::RenderTexture& texture);
 void win_draw_string(sf::RenderTarget& dest_window,rectangle dest_rect,std::string str,eTextMode mode,TextStyle style,bool right_align = false);
 void win_draw_string(sf::RenderTarget& dest_window,rectangle dest_rect,std::string str,eTextMode mode,TextStyle style, break_info_t break_info,bool right_align = false);
 break_info_t calculate_line_wrapping(rectangle dest_rect, std::string str, TextStyle style);
-size_t string_length(std::string str, TextStyle style, short* height = nullptr);
+size_t string_length(std::string str, const TextStyle& style, short* height = nullptr);
 
 #endif