WindowServer: Use FB_IOCTL_FLUSH_HEAD to flush a framebuffer if possible

This ioctl is more appropriate when the hardware supports flushing of
the entire framebuffer, so we use that instead of the previous default
FB_IOCTL_FLUSH_HEAD_BUFFERS ioctl.

Userland/Services/WindowServer/VirtualScreenBackend.h

@@ -30,6 +30,8 @@ private:
 
     virtual ErrorOr<void> flush_framebuffer_rects(int, Span<FBRect const>) override { return {}; }
 
+    virtual ErrorOr<void> flush_framebuffer() override { return {}; }
+
     virtual ErrorOr<void> unmap_framebuffer() override;
     virtual ErrorOr<void> map_framebuffer() override;