LibGfx+LibWeb: Move Gfx::ScaledFont caching from LibWeb into LibGfx

Before this change, we would only cache and reuse Gfx::ScaledFont
instances for downloaded CSS fonts.

By moving it into Gfx::VectorFont, we get caching for all vector fonts,
including local system TTFs etc.

This avoids a *lot* of style invalidations in LibWeb, since we now vend
the same Gfx::Font pointer for the same font when used repeatedly.

Userland/Libraries/LibGfx/Font/VectorFont.cpp

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/Font/ScaledFont.h>
+#include <LibGfx/Font/VectorFont.h>
+
+namespace Gfx {
+
+VectorFont::VectorFont() = default;
+VectorFont::~VectorFont() = default;
+
+NonnullRefPtr<ScaledFont> VectorFont::scaled_font(float point_size) const
+{
+    auto it = m_scaled_fonts.find(point_size);
+    if (it != m_scaled_fonts.end())
+        return *it->value;
+
+    // FIXME: It might be nice to have a global cap on the number of fonts we cache
+    //        instead of doing it at the per-VectorFont level like this.
+    constexpr size_t max_cached_font_size_count = 128;
+    if (m_scaled_fonts.size() > max_cached_font_size_count)
+        m_scaled_fonts.remove(m_scaled_fonts.begin());
+
+    auto scaled_font = adopt_ref(*new ScaledFont(*this, point_size, point_size));
+    m_scaled_fonts.set(point_size, scaled_font);
+    return scaled_font;
+}
+}