LibHTML: Add a simple font cache

The FontCache caches the result of font lookups. The cache key is a
simple object called FontSelector which consists of the font family
and font weight (both strings.)

This drastically reduces time spent in font lookup.

Libraries/LibHTML/FontCache.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include <AK/HashMap.h>
+#include <AK/String.h>
+
+class Font;
+
+struct FontSelector {
+    String family;
+    String weight;
+
+    bool operator==(const FontSelector& other) const
+    {
+        return family == other.family && weight == other.weight;
+    }
+};
+
+namespace AK {
+template<>
+struct Traits<FontSelector> : public GenericTraits<FontSelector> {
+    static unsigned hash(const FontSelector& key) { return pair_int_hash(key.family.hash(), key.weight.hash()); }
+};
+}
+
+class FontCache {
+public:
+    static FontCache& the();
+    RefPtr<Font> get(const FontSelector&) const;
+    void set(const FontSelector&, NonnullRefPtr<Font>);
+
+private:
+    FontCache() {}
+    mutable HashMap<FontSelector, NonnullRefPtr<Font>> m_fonts;
+};