RHI RenderCapture capture results differ from OpenGL renderer

QRenderCapture behaves differently when using the RHI renderer than when using the OpenGL renderer.

• The OpenGL renderer only captures the rendered content of the 3D scene, while the RHI renderer captures the entire root QML item.
• The 'rect' argument of the requestCapture function does not work with the RHI renderer.

Step to reproduce:

1. Apply rendercapture-qml.patch to qt3d/tests/manual/rendercapture-qml
2. Build and run the rendercapture-qml

The entire loot item is captured, including the red circle

Using the OpenGL renderer with environment variables QSG_RHI_BACKEND=opengl and QT3D_RENDERER=opengl works as expected.