rhi: Make QRhiRenderTarget's rp getter functional with swapchains

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>

src/gui/rhi/qrhid3d11.cpp

@@ -4827,6 +4827,7 @@ bool QD3D11SwapChain::createOrResize()
     frameCount = 0;
     ds = m_depthStencil ? QRHI_RES(QD3D11RenderBuffer, m_depthStencil) : nullptr;
 
+    rt.setRenderPassDescriptor(m_renderPassDesc); // for the public getter in QRhiRenderTarget
     QD3D11SwapChainRenderTarget *rtD = QRHI_RES(QD3D11SwapChainRenderTarget, &rt);
     rtD->d.rp = QRHI_RES(QD3D11RenderPassDescriptor, m_renderPassDesc);
     rtD->d.pixelSize = pixelSize;

src/gui/rhi/qrhigles2.cpp

@@ -5795,6 +5795,7 @@ bool QGles2SwapChain::createOrResize()
         m_depthStencil->create();
     }
 
+    rt.setRenderPassDescriptor(m_renderPassDesc); // for the public getter in QRhiRenderTarget
     rt.d.rp = QRHI_RES(QGles2RenderPassDescriptor, m_renderPassDesc);
     rt.d.pixelSize = pixelSize;
     rt.d.dpr = float(m_window->devicePixelRatio());

src/gui/rhi/qrhimetal.mm

@@ -4179,6 +4179,7 @@ bool QMetalSwapChain::createOrResize()
         }
     }
 
+    rtWrapper.setRenderPassDescriptor(m_renderPassDesc); // for the public getter in QRhiRenderTarget
     rtWrapper.d->pixelSize = pixelSize;
     rtWrapper.d->dpr = float(window->devicePixelRatio());
     rtWrapper.d->sampleCount = samples;

src/gui/rhi/qrhinull.cpp

@@ -1015,6 +1015,7 @@ bool QNullSwapChain::createOrResize()
 
     window = m_window;
     m_currentPixelSize = surfacePixelSize();
+    rt.setRenderPassDescriptor(m_renderPassDesc); // for the public getter in QRhiRenderTarget
     rt.d.rp = QRHI_RES(QNullRenderPassDescriptor, m_renderPassDesc);
     rt.d.pixelSize = m_currentPixelSize;
     frameCount = 0;

src/gui/rhi/qrhivulkan.cpp

@@ -7507,6 +7507,7 @@ bool QVkSwapChain::createOrResize()
     if (!m_renderPassDesc)
         qWarning("QVkSwapChain: No renderpass descriptor set. See newCompatibleRenderPassDescriptor() and setRenderPassDescriptor().");
 
+    rtWrapper.setRenderPassDescriptor(m_renderPassDesc); // for the public getter in QRhiRenderTarget
     rtWrapper.d.rp = QRHI_RES(QVkRenderPassDescriptor, m_renderPassDesc);
     Q_ASSERT(rtWrapper.d.rp && rtWrapper.d.rp->rp);
 

tests/auto/gui/rhi/qrhi/tst_qrhi.cpp

@@ -3456,6 +3456,7 @@ void tst_QRhi::renderToWindowSimple()
         QRhiCommandBuffer *cb = swapChain->currentFrameCommandBuffer();
         QRhiRenderTarget *rt = swapChain->currentFrameRenderTarget();
         QCOMPARE(rt->resourceType(), QRhiResource::SwapChainRenderTarget);
+        QVERIFY(rt->renderPassDescriptor());
         QCOMPARE(static_cast<QRhiSwapChainRenderTarget *>(rt)->swapChain(), swapChain.data());
         const QSize outputSize = swapChain->currentPixelSize();
         QCOMPARE(rt->pixelSize(), outputSize);