Everywhere: Use IOSurface as backing store on macOS

Using mmap-allocated memory for backing stores does not allow us to
benefit from using GPU-accelerated painting, because all the performance
increase we get is mostly negated by reading the GPU-allocated texture
back into RAM, so it can be shared with the browser process.

With IOSurface, we get a framebuffer that is both shareable between
processes and can be used as underlying memory for an OpenGL/Metal
texture.

This change does not yet benefit from using IOSurface and merely wraps
them into Gfx::Bitmap to be used by the CPU painter.

Userland/Libraries/LibWebView/ViewImplementation.h

@@ -12,6 +12,7 @@
 #include <AK/LexicalPath.h>
 #include <AK/Queue.h>
 #include <AK/String.h>
+#include <LibCore/Forward.h>
 #include <LibCore/Promise.h>
 #include <LibGfx/Forward.h>
 #include <LibGfx/StandardCursor.h>
@@ -118,6 +119,9 @@ public:
     void did_update_navigation_buttons_state(Badge<WebContentClient>, bool back_enabled, bool forward_enabled) const;
 
     void did_allocate_backing_stores(Badge<WebContentClient>, i32 front_bitmap_id, Gfx::ShareableBitmap const&, i32 back_bitmap_id, Gfx::ShareableBitmap const&);
+#ifdef AK_OS_MACOS
+    void did_allocate_iosurface_backing_stores(i32 front_bitmap_id, Core::MachPort&&, i32 back_bitmap_id, Core::MachPort&&);
+#endif
 
     enum class ScreenshotType {
         Visible,