[ChangeLog][Renderer] Optionally render to the scenegraph framebuffer

Change-Id: Ibd0b8b5e6524e91ca1d576975dc353fe49e44d54
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>