rhi: vulkan: Expose the VkRenderPass via the usual mechanisms

Qt Quick in turn will expose it via QSGRendererInterface. Essential when
adding custom Vulkan rendering into a Qt Quick application because the
custom pipeline state objects will need to reference a VkRenderPass.

Change-Id: Idf4092cfc3937830fb8123164081059b0d8d030e
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/gui/rhi/qrhi.cpp

@@ -2246,6 +2246,10 @@ QRhiResource::Type QRhiSampler::resourceType() const
     \internal
     \inmodule QtGui
     \brief Render pass resource.
+
+    A render pass, if such a concept exists in the underlying graphics API, is
+    a collection of attachments (color, depth, stencil) and describes how those
+    attachments are used.
  */
 
 /*!
@@ -2264,6 +2268,18 @@ QRhiResource::Type QRhiRenderPassDescriptor::resourceType() const
     return RenderPassDescriptor;
 }
 
+/*!
+    \return a pointer to a backend-specific QRhiNativeHandles subclass, such as
+    QRhiVulkanRenderPassNativeHandles. The returned value is null when exposing
+    the underlying native resources is not supported by the backend.
+
+    \sa QRhiVulkanRenderPassNativeHandles
+ */
+const QRhiNativeHandles *QRhiRenderPassDescriptor::nativeHandles()
+{
+    return nullptr;
+}
+
 /*!
     \class QRhiRenderTarget
     \internal

src/gui/rhi/qrhi_p.h

@@ -843,6 +843,8 @@ class Q_GUI_EXPORT QRhiRenderPassDescriptor : public QRhiResource
 public:
     QRhiResource::Type resourceType() const override;
 
+    virtual const QRhiNativeHandles *nativeHandles();
+
 protected:
     QRhiRenderPassDescriptor(QRhiImplementation *rhi);
 };

src/gui/rhi/qrhivulkan.cpp

@@ -182,6 +182,13 @@ QT_BEGIN_NAMESPACE
     \l{QRhi::endOffsrceenFrame()}{endOffscreenFrame()} pair.
  */
 
+/*!
+    \class QRhiVulkanRenderPassNativeHandles
+    \internal
+    \inmodule QtGui
+    \brief Holds the Vulkan render pass object backing a QRhiRenderPassDescriptor.
+ */
+
 static inline VkDeviceSize aligned(VkDeviceSize v, VkDeviceSize byteAlign)
 {
     return (v + byteAlign - 1) & ~(byteAlign - 1);
@@ -5098,6 +5105,12 @@ void QVkRenderPassDescriptor::release()
     rhiD->unregisterResource(this);
 }
 
+const QRhiNativeHandles *QVkRenderPassDescriptor::nativeHandles()
+{
+    nativeHandlesStruct.renderPass = rp;
+    return &nativeHandlesStruct;
+}
+
 QVkReferenceRenderTarget::QVkReferenceRenderTarget(QRhiImplementation *rhi)
     : QRhiRenderTarget(rhi)
 {

src/gui/rhi/qrhivulkan_p.h

@@ -80,6 +80,11 @@ struct Q_GUI_EXPORT QRhiVulkanCommandBufferNativeHandles : public QRhiNativeHand
     VkCommandBuffer commandBuffer = VK_NULL_HANDLE;
 };
 
+struct Q_GUI_EXPORT QRhiVulkanRenderPassNativeHandles : public QRhiNativeHandles
+{
+    VkRenderPass renderPass = VK_NULL_HANDLE;
+};
+
 QT_END_NAMESPACE
 
 #endif

src/gui/rhi/qrhivulkan_p_p.h

@@ -171,9 +171,11 @@ struct QVkRenderPassDescriptor : public QRhiRenderPassDescriptor
     QVkRenderPassDescriptor(QRhiImplementation *rhi);
     ~QVkRenderPassDescriptor();
     void release() override;
+    const QRhiNativeHandles *nativeHandles() override;
 
     VkRenderPass rp = VK_NULL_HANDLE;
     bool ownsRp = false;
+    QRhiVulkanRenderPassNativeHandles nativeHandlesStruct;
     int lastActiveFrameSlot = -1;
 };