LibWebView+WebContent: Move backing store allocation into WebContent

In the upcoming changes, we are going to switch macOS to using an
IOSurface for the backing store. This change will simplify the process
of sharing an IOSurface between processes because we already have the
MachPortServer running in the browser, and WebContent knows how to
locate the corresponding server.

Userland/Services/WebContent/BackingStoreManager.cpp

@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2024, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/Timer.h>
+#include <LibWeb/HTML/TraversableNavigable.h>
+#include <WebContent/BackingStoreManager.h>
+#include <WebContent/PageClient.h>
+
+namespace WebContent {
+
+BackingStoreManager::BackingStoreManager(PageClient& page_client)
+    : m_page_client(page_client)
+{
+    m_backing_store_shrink_timer = Core::Timer::create_single_shot(3000, [this] {
+        resize_backing_stores_if_needed(WindowResizingInProgress::No);
+    });
+}
+
+void BackingStoreManager::restart_resize_timer()
+{
+    m_backing_store_shrink_timer->restart();
+}
+
+void BackingStoreManager::resize_backing_stores_if_needed(WindowResizingInProgress window_resize_in_progress)
+{
+    auto css_pixels_viewport_rect = m_page_client.page().top_level_traversable()->viewport_rect();
+    auto viewport_size = m_page_client.page().css_to_device_rect(css_pixels_viewport_rect).size();
+
+    if (viewport_size.is_empty())
+        return;
+
+    Web::DevicePixelSize minimum_needed_size;
+    if (window_resize_in_progress == WindowResizingInProgress::Yes) {
+        // Pad the minimum needed size so that we don't have to keep reallocating backing stores while the window is being resized.
+        minimum_needed_size = { viewport_size.width() + 256, viewport_size.height() + 256 };
+    } else {
+        // If we're not in the middle of a resize, we can shrink the backing store size to match the viewport size.
+        minimum_needed_size = viewport_size;
+        m_front_bitmap.clear();
+        m_back_bitmap.clear();
+    }
+
+    auto old_front_bitmap_id = m_front_bitmap_id;
+    auto old_back_bitmap_id = m_back_bitmap_id;
+
+    auto reallocate_backing_store_if_needed = [&](RefPtr<Gfx::Bitmap>& bitmap, int& id) {
+        if (!bitmap || !bitmap->size().contains(minimum_needed_size.to_type<int>())) {
+            if (auto new_bitmap_or_error = Gfx::Bitmap::create_shareable(Gfx::BitmapFormat::BGRA8888, minimum_needed_size.to_type<int>()); !new_bitmap_or_error.is_error()) {
+                bitmap = new_bitmap_or_error.release_value();
+                id = m_next_bitmap_id++;
+            }
+        }
+    };
+
+    reallocate_backing_store_if_needed(m_front_bitmap, m_front_bitmap_id);
+    reallocate_backing_store_if_needed(m_back_bitmap, m_back_bitmap_id);
+
+    auto& front_bitmap = m_front_bitmap;
+    auto& back_bitmap = m_back_bitmap;
+
+    if (m_front_bitmap_id != old_front_bitmap_id || m_back_bitmap_id != old_back_bitmap_id) {
+        m_page_client.page_did_allocate_backing_stores(m_front_bitmap_id, front_bitmap->to_shareable_bitmap(), m_back_bitmap_id, back_bitmap->to_shareable_bitmap());
+    }
+}
+
+void BackingStoreManager::swap_back_and_front()
+{
+    swap(m_front_bitmap, m_back_bitmap);
+    swap(m_front_bitmap_id, m_back_bitmap_id);
+}
+
+}