gsk: Add a way to get a scaled font

Add a function to change the size of a font while keeping everything
else the same. We use pango api for this if available.

gsk/gskprivate.c

@@ -1,6 +1,10 @@
 #include "gskresources.h"
 #include "gskprivate.h"
 
+#include <cairo.h>
+#include <pango/pangocairo.h>
+#include <math.h>
+
 static gpointer
 register_resources (gpointer data)
 {
@@ -15,3 +19,78 @@ gsk_ensure_resources (void)
 
   g_once (&register_resources_once, register_resources, NULL);
 }
+
+/*< private >
+ * gsk_get_scaled_font:
+ * @font: a `PangoFont`
+ * @scale: the scale
+ *
+ * Returns a font that is just like @font, at a size that
+ * is multiplied by @scale.
+ *
+ * Returns: (transfer full): a scaled version of @font
+ */
+PangoFont *
+gsk_get_scaled_font (PangoFont *font,
+                     float      scale)
+{
+  if (scale == 1.0)
+    return g_object_ref (font);
+
+#if PANGO_VERSION_CHECK (1, 52, 0)
+  return pango_font_map_reload_font (pango_font_get_font_map (font), font, scale, NULL, NULL);
+#else
+  GHashTable *fonts;
+  int key;
+  PangoFont *font2;
+  PangoFontDescription *desc;
+  int size;
+  PangoFontMap *fontmap;
+  PangoContext *context;
+  cairo_scaled_font_t *sf;
+  cairo_font_options_t *options;
+
+  key = (int) roundf (scale * PANGO_SCALE);
+
+  fonts = (GHashTable *) g_object_get_data (G_OBJECT (font), "gsk-scaled-fonts");
+
+  if (fonts)
+    {
+      font2 = g_hash_table_lookup (fonts, GINT_TO_POINTER (key));
+      if (font2)
+        return g_object_ref (font2);
+    }
+  else
+    {
+      fonts = g_hash_table_new_full (g_direct_hash, g_direct_equal, NULL, g_object_unref);
+      g_object_set_data_full (G_OBJECT (font), "gsk-scaled-fonts",
+                              fonts, (GDestroyNotify) g_hash_table_unref);
+    }
+
+  desc = pango_font_describe (font);
+  size = pango_font_description_get_size (desc);
+
+  if (pango_font_description_get_size_is_absolute (desc))
+    pango_font_description_set_absolute_size (desc, size * scale);
+  else
+    pango_font_description_set_size (desc, (int) roundf (size * scale));
+
+  fontmap = pango_font_get_font_map (font);
+  context = pango_font_map_create_context (fontmap);
+
+  sf = pango_cairo_font_get_scaled_font (PANGO_CAIRO_FONT (font));
+  options = cairo_font_options_create ();
+  cairo_scaled_font_get_font_options (sf, options);
+  pango_cairo_context_set_font_options (context, options);
+  cairo_font_options_destroy (options);
+
+  font2 = pango_font_map_load_font (fontmap, context, desc);
+
+  pango_font_description_free (desc);
+  g_object_unref (context);
+
+  g_hash_table_insert (fonts, GINT_TO_POINTER (key), font2);
+
+  return g_object_ref (font2);
+#endif
+}