QQuickWidget: Play nice with Vulkan and offscreen grabbing

The texture does not have the UsedAsTransferSource flag set. With
Vulkan this results in a validation layer warning (if that is enabled)
when issuing the VkImage -> buffer copy during the grab. Add the flag.

We also need to ensure there is a QVulkanInstance
(VkInstance). Normally we do not care about this and just forward what
the toplevel/backingstore use, but this is not there when trying to
grab with a not-yet-shown widget.  Use the helper singleton
(QVulkanDefaultInstance) to get a QVulkanInstance. (this is also what
the backingstore and Qt Quick uses)

Change-Id: I9f5b4a67a1a7862e6b89e76f2a6d55c8a2be1972
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit b676c3fbc7117fbf0b3580c16a1173f9c383d54c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/quickwidgets/qquickwidget.cpp b/src/quickwidgets/qquickwidget.cpp
index 205c9e31c7..a0efe916da 100644
--- a/src/quickwidgets/qquickwidget.cpp
+++ b/src/quickwidgets/qquickwidget.cpp
@@ -47,6 +47,10 @@
 #include <QtWidgets/qgraphicsview.h>
 #endif
 
+#if QT_CONFIG(vulkan)
+#include <QtGui/private/qvulkandefaultinstance_p.h>
+#endif
+
 QT_BEGIN_NAMESPACE
 
 QQuickWidgetOffscreenWindow::QQuickWidgetOffscreenWindow(QQuickWindowPrivate &dd, QQuickRenderControl *control)
@@ -1084,6 +1088,8 @@ void QQuickWidgetPrivate::initializeWithRhi()
 #if QT_CONFIG(vulkan)
             if (QWindow *w = q->window()->windowHandle())
                 offscreenWindow->setVulkanInstance(w->vulkanInstance());
+            else if (rhi == offscreenRenderer.rhi())
+                offscreenWindow->setVulkanInstance(QVulkanDefaultInstance::instance());
 #endif
             renderControl->initialize();
         }
@@ -1130,7 +1136,7 @@ void QQuickWidget::createFramebufferObject()
 
     // Could be a simple hide - show, in which case the previous texture is just fine.
     if (!d->outputTexture) {
-        d->outputTexture = d->rhi->newTexture(QRhiTexture::RGBA8, fboSize, 1, QRhiTexture::RenderTarget);
+        d->outputTexture = d->rhi->newTexture(QRhiTexture::RGBA8, fboSize, 1, QRhiTexture::RenderTarget | QRhiTexture::UsedAsTransferSource);
         if (!d->outputTexture->create()) {
             qWarning("QQuickWidget: failed to create output texture of size %dx%d",
                      fboSize.width(), fboSize.height());