font: Reimplement font::get_max_height() using Pango

A few caveats:

 * This implementation allows configuring the font family class and
   style to select the correct font for any given context. The default
   values are equivalent to what was being done in the SDL_ttf-based
   version, but not future-proof. Users like the GUI2 textbox should
   make use of the new optional parameters to select a more relevant
   font going forward (not part of this commit).

 * Pango has a line height metric, equivalent to SDL_ttf's
   TTF_FontLineSkip(). However, this is not what we want here -- the
   ascender + descender formula is intended to give us a neat box where
   the tallest glyph can fit, not the minimum distance between
   baselines. This is important for the GUI2 textbox in particular.

 * The internal implementation uses the default text renderer and sets
   relevant parameters. This should not introduce new issues, but if it
   does then that means someone is not resetting the renderer properly
   before using it.

This notably rids GUI2 of its one known dependency on SDL_ttf.

src/font/sdl_ttf.cpp

@@ -352,15 +352,6 @@ SDL_Rect draw_text_line(surface& gui_surface, const SDL_Rect& area, int size,
 	return dest;
 }
 
-int get_max_height(int size)
-{
-	// Only returns the maximal size of the first font
-	const auto font = sdl_ttf::get_font(font_id(0, size));
-	if(font == nullptr)
-		return 0;
-	return TTF_FontHeight(font.get());
-}
-
 int line_width(const std::string& line, int font_size, int style)
 {
 	return line_size(line,font_size,style).w;

src/font/sdl_ttf.hpp

@@ -35,9 +35,6 @@ SDL_Rect draw_text_line(surface& gui_surface, const SDL_Rect& area, int size,
 						const color_t& color, const std::string& text,
 						int x, int y, bool use_tooltips, int style);
 
-// Returns the maximum height of a font, in pixels
-int get_max_height(int size);
-
 /**
  * Determine the width of a line of text given a certain font size.
  * The font type used is the default wesnoth font type.

src/font/text.cpp

@@ -477,6 +477,26 @@ pango_text& pango_text::set_add_outline(bool do_add)
 	return *this;
 }
 
+int pango_text::get_max_glyph_height() const
+{
+	p_font font{ get_font_families(font_class_), font_size_, font_style_ };
+
+	PangoFont* f = pango_font_map_load_font(
+		pango_cairo_font_map_get_default(),
+		context_.get(),
+		font.get());
+
+	PangoFontMetrics* m = pango_font_get_metrics(f, nullptr);
+
+	auto ascent = pango_font_metrics_get_ascent(m);
+	auto descent = pango_font_metrics_get_descent(m);
+
+	pango_font_metrics_unref(m);
+	g_object_unref(f);
+
+	return ceil(pango_units_to_double(ascent + descent));
+}
+
 void pango_text::recalculate(const bool force) const
 {
 	if(calculation_dirty_ || force) {
@@ -900,4 +920,14 @@ pango_text& get_text_renderer()
 	return text_renderer;
 }
 
+int get_max_height(unsigned size, font::family_class fclass, pango_text::FONT_STYLE style)
+{
+	// Reset metrics to defaults
+	return get_text_renderer()
+		.set_family_class(fclass)
+		.set_font_style(style)
+		.set_font_size(size)
+		.get_max_glyph_height();
+}
+
 } // namespace font

src/font/text.hpp

@@ -143,6 +143,15 @@ class pango_text
 
 	/***** ***** ***** ***** Query details ***** ***** ***** *****/
 
+	/**
+	 * Returns the maximum glyph height of a font, in pixels.
+	 *
+	 * @returns                       The height of the tallest possible glyph for the selected
+	 *                                font. More specifically, the result is the sum of the maximum
+	 *                                ascent and descent lengths.
+	 */
+	int get_max_glyph_height() const;
+
 	/**
 	 * Gets the location for the cursor.
 	 *
@@ -431,4 +440,17 @@ class pango_text
  */
 pango_text& get_text_renderer();
 
+/**
+ * Returns the maximum glyph height of a font, in pixels.
+ *
+ * @param size                    Desired font size in pixels.
+ * @param fclass                  Font family to use for measurement.
+ * @param style                   Font style to select the correct variant for measurement.
+ *
+ * @returns                       The height of the tallest possible glyph for the selected
+ *                                font. More specifically, the result is the sum of the maximum
+ *                                ascent and descent lengths.
+ */
+int get_max_height(unsigned size, font::family_class fclass = font::FONT_SANS_SERIF, pango_text::FONT_STYLE style = pango_text::STYLE_NORMAL);
+
 } // namespace font