swapchain->currentFrameRenderTarget()->renderPassDescriptor() is not
functional at the moment, it returns null. This is because no backend
ensures that the internal renderpass descriptor object is exposed via
that getter in a QRhiSwapChainRenderTarget. Whereas in a
QRhiTextureRenderTarget this would work by design because there the
setter must be called by the user.
Fix this up, providing better API symmetry, and also reducing the need
to pass along QRhiRenderPassDescriptor objects seprately alongside a
QRhiRenderTarget in some places, e.g. in Qt Quick.
Change-Id: I42c4e9aaee3202c1d23bd093d840af80c5f8cd0f
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
b78f3f4974