Make QQuickFbo work with the OpenGL backend of QRhi

So no matter if Quick goes directly to OpenGL, or via QRhi,
QQuickFramebufferObject will still work.

Also fix up the fboitem example to use a ShaderEffect that works with
both rendering paths.

With graphics APIs other than OpenGL the item will be empty, as QQuickFbo
is not something we can support there.

Task-number: QTBUG-79222
Change-Id: I52177d3a75f619f7075a2fc829573c17031eded1
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 9 insertions, 4 deletions

diff --git a/src/quick/items/qquickwindow.cpp b/src/quick/items/qquickwindow.cpp
index 6286a1694a..905241989e 100644
--- a/src/quick/items/qquickwindow.cpp
+++ b/src/quick/items/qquickwindow.cpp
@@ -4735,8 +4735,7 @@ void QQuickWindow::setDefaultAlphaBuffer(bool useAlpha)
     adaptation.
 
     \note This function has no effect when running on the RHI graphics
-    abstraction. With the RHI, the functions to call when enqueuing native
-    graphics commands are beginExternalCommands() and endExternalCommands().
+    abstraction and the underlying RHI backend is not OpenGL.
 
     \sa QQuickWindow::beforeRendering(), beginExternalCommands(), endExternalCommands()
  */
@@ -4744,7 +4743,7 @@ void QQuickWindow::resetOpenGLState()
 {
     Q_D(QQuickWindow);
 
-    if (d->rhi || !openglContext())
+    if (!openglContext())
         return;
 
     QOpenGLContext *ctx = openglContext();
@@ -4859,7 +4858,13 @@ const QQuickWindow::GraphicsStateInfo &QQuickWindow::graphicsStateInfo()
     directly and the RHI graphics abstraction layer is not in use. Refer to
     resetOpenGLState() in that case.
 
-    \sa endExternalCommands()
+    \note When the scenegraph is using the RHI graphics abstraction layer with
+    the OpenGL backend underneath, pay attention to the fact that the OpenGL
+    state in the context can have arbitrary settings, and this function does not
+    perform any resetting of the state back to defaults. Call
+    resetOpenGLState() if that is seen necessary.
+
+    \sa endExternalCommands(), resetOpenGLState()
 
     \since 5.14
  */