Only alter rhi backend on Win7 and Metalless-macOS when not explicitly requested

When forcing a given backend from the C++ API or QSG_RHI_BACKEND, the
user/application's request must be honored. Otherwise confusing
situations arise. If the requested backend fails, let's just fail.

It is the "just pick a platform specific default" case where we have the
flexibility of choosing between the backends.

Change-Id: If30c329bc7ce8ec26abee0ae2d4bcaef53df4718
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/quick/scenegraph/qsgrhisupport.cpp b/src/quick/scenegraph/qsgrhisupport.cpp
index fe3b626077..22a1827f72 100644
--- a/src/quick/scenegraph/qsgrhisupport.cpp
+++ b/src/quick/scenegraph/qsgrhisupport.cpp
@@ -177,14 +177,17 @@ void QSGRhiSupport::applySettings()
             m_rhiBackend = QRhi::OpenGLES2;
 #endif
             // Vulkan has to be requested explicitly
+
+            // Now that we established our initial choice, we may want to opt
+            // for another backend under certain special circumstances.
+            if (m_enableRhi) // guard because this may do actual graphics calls on some platforms
+                adjustToPlatformQuirks();
         }
     }
 
     if (!m_enableRhi)
         return;
 
-    adjustToPlatformQuirks();
-
     // At this point the RHI backend is fixed, it cannot be changed once we
     // return from this function. This is because things like the QWindow
     // (QQuickWindow) may depend on the graphics API as well (surfaceType