misc: don't prefer Vulkan RHI

WebEngine view destruction crashes on Windows 10 when using Vulkan. Disable the preference for now. It will default to D3D11.

Gitlab: #1515
Change-Id: I8396163a937b3b439569115887020d49e3a8e751

CMakeLists.txt

@@ -343,6 +343,8 @@ if(Vulkan_FOUND)
   add_definitions(-DHAS_VULKAN)
   include_directories(${Vulkan_INCLUDE_DIR})
 endif()
+# Define PREFER_VULKAN to prefer Vulkan over the default API
+# on GNU/Linux and Windows. Metal is always preferred on macOS.
 
 if(MSVC)
   set(WINDOWS_SYS_LIBS

src/app/main.cpp

@@ -93,7 +93,7 @@ main(int argc, char* argv[])
     }
 #else
     if (std::invoke([] {
-#if defined(HAS_VULKAN) && !defined(Q_OS_LINUX)
+#if defined(HAS_VULKAN) && defined(PREFER_VULKAN)
             // Somehow, several bug reports show that, on Windows, QVulkanInstance
             // verification  passes, but goes on to fail when creating the QQuickWindow
             // with "Failed to initialize graphics backend for Vulkan".
@@ -119,8 +119,6 @@ main(int argc, char* argv[])
         // removed. So we need to re-implement this (custom controls)
         // or wait for a future version
         QQuickWindow::setGraphicsApi(QSGRendererInterface::VulkanRhi);
-    } else {
-        QQuickWindow::setGraphicsApi(QSGRendererInterface::Unknown);
     }
 #endif