QSGRenderNode needs a prepare() function (or at minimum, an internal callback) to properly support the typical two-phase prepare-render approach

QTBUG-82793 shows that Qt Quick 3D is unable to implement its behavior for the Inline View3D render mode because QSGRenderNode only has a render() virtual, whereas Quick3D would need access to the matrix() already in the prepare step, and there is no suitable virtual in QSGRenderNode atm.

Also needs a flag to indicate whether QRhiCommandBuffer::beginExternal() should be called. (because if the render() implementation is using QRhi then the answer is NO)

This will be done in two parts:

• Quick3D can live with using QSGRenderNodePrivate. So will just add a callback there.
• Once the dev branches are usable, this can be changed to a virtual prepare() function in QSGRenderNode.