Conclude how to deal with the more up front nature of the QRhi API in Qt Quick

May need an API on QQuickWindow or elsewhere to indicate that external native rendering will be used (overlay, underlay, rendernode, ...) ExternalContentsInPass is a beginFrame() flag in QRhi, but we cannot guess this in Qt Quick, and therefore we either set it always or never.

For now we assume yes always, even though this is not ideal strictly speaking since it triggers secondary command buffer usage for all Qt Quick apps when running on Vulkan, including those that do not integrate their own custom rendering, which is harmless, but is still extra work at run time.

The case itself is not necessarily that interesting, but how we handle it should define a pattern for possible future cases that are similar to this one. With some of the underlying graphics APIs leaning heavily towards declaring everything (how a resource is used etc.) up front we cannot guarantee that similar flags will not be introduced later on.