Avoid surface copying when rendering text

This was a small proposed change as part of PR #844. While that PR is rather outdated at this point, this seemed like
a simple, useful change.

src/font/text.cpp

@@ -100,7 +100,7 @@ pango_text::~pango_text()
 	surface_.assign(nullptr);
 }
 
-surface pango_text::render() const
+surface& pango_text::render() const
 {
 	this->rerender();
 	return surface_;

src/font/text.hpp

@@ -90,7 +90,7 @@ public:
 	 * Before rendering it tests whether a redraw is needed and if so it first
 	 * redraws the surface before returning it.
 	 */
-	surface render() const;
+	surface& render() const;
 
 	/** Returns the width needed for the text. */
 	int get_width() const;

src/gui/core/canvas.cpp

@@ -1346,7 +1346,7 @@ void text_shape::draw(surface& canvas,
 							 : PANGO_ELLIPSIZE_END)
 			.set_characters_per_line(characters_per_line_);
 
-	surface surf = text_renderer.render();
+	surface& surf = text_renderer.render();
 	if(surf->w == 0) {
 		DBG_GUI_D << "Text: Rendering '" << text
 				  << "' resulted in an empty canvas, leave.\n";