LibGfx/OpenType: Cache kerning values for faster text layout

The Font class now remembers the results of kerning lookups in a
HashMap. This fixes an issue where text-heavy UI (like WidgetGallery)
would lag when using a UI font with kerning data.

Userland/Libraries/LibGfx/Font/OpenType/Font.cpp

@@ -676,15 +676,30 @@ Gfx::ScaledGlyphMetrics Font::glyph_metrics(u32 glyph_id, float x_scale, float y
 
 float Font::glyphs_horizontal_kerning(u32 left_glyph_id, u32 right_glyph_id, float x_scale) const
 {
+    if (!m_gpos.has_value() && !m_kern.has_value())
+        return 0.0f;
+
+    // NOTE: OpenType glyph IDs are 16-bit, so this is safe.
+    auto cache_key = (left_glyph_id << 16) | right_glyph_id;
+    if (auto it = m_kerning_cache.find(cache_key); it != m_kerning_cache.end()) {
+        return it->value * x_scale;
+    }
+
     if (m_gpos.has_value()) {
         auto kerning = m_gpos->glyph_kerning(left_glyph_id, right_glyph_id);
-        if (kerning.has_value())
+        if (kerning.has_value()) {
+            m_kerning_cache.set(cache_key, kerning.value());
             return kerning.value() * x_scale;
+        }
     }
 
-    if (m_kern.has_value())
-        return m_kern->get_glyph_kerning(left_glyph_id, right_glyph_id) * x_scale;
+    if (m_kern.has_value()) {
+        auto kerning = m_kern->get_glyph_kerning(left_glyph_id, right_glyph_id) * x_scale;
+        m_kerning_cache.set(cache_key, kerning);
+        return kerning;
+    }
 
+    m_kerning_cache.set(cache_key, 0);
     return 0.0f;
 }
 

Userland/Libraries/LibGfx/Font/OpenType/Font.h

@@ -141,6 +141,8 @@ private:
 
     mutable HashMap<size_t, NonnullOwnPtr<GlyphPage>> m_glyph_pages;
 
+    mutable HashMap<u32, i16> m_kerning_cache;
+
     GlyphPage const& glyph_page(size_t page_index) const;
     void populate_glyph_page(GlyphPage&, size_t page_index) const;
 };