Enable QWindowContainer on Android

[ChangeLog][Platform Specific Changes][Android] QWidget::createWindowContainer()
is now supported on Android for embedding OpenGL-based QWindows into widget UIs.

Task-number: QTBUG-59175
Change-Id: I84d3703bcd44c63b1fdfe6772b7f3de9d5c18ddf
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

src/gui/kernel/qplatformintegration.cpp

@@ -237,6 +237,10 @@ QPlatformServices *QPlatformIntegration::services() const
     is required to have this capability.
 
     \value ApplicationIcon The platform supports setting the application icon. (since 5.5)
+
+    \value TopStackedNativeChildWindows The platform supports native child windows via
+    QWindowContainer without having to punch a transparent hole in the
+    backingstore. (since 5.10)
  */
 
 /*!
@@ -260,7 +264,8 @@ QPlatformServices *QPlatformIntegration::services() const
 
 bool QPlatformIntegration::hasCapability(Capability cap) const
 {
-    return cap == NonFullScreenWindows || cap == NativeWidgets || cap == WindowManagement;
+    return cap == NonFullScreenWindows || cap == NativeWidgets || cap == WindowManagement
+        || cap == TopStackedNativeChildWindows;
 }
 
 QPlatformPixmap *QPlatformIntegration::createPlatformPixmap(QPlatformPixmap::PixelType type) const

src/gui/kernel/qplatformintegration.h

@@ -99,7 +99,8 @@ public:
         RasterGLSurface,
         AllGLFunctionsQueryable,
         ApplicationIcon,
-        SwitchableWidgetComposition
+        SwitchableWidgetComposition,
+        TopStackedNativeChildWindows
     };
 
     virtual ~QPlatformIntegration() { }

src/plugins/platforms/android/qandroidplatformintegration.cpp

@@ -233,6 +233,7 @@ bool QAndroidPlatformIntegration::hasCapability(Capability cap) const
         case ForeignWindows: return QtAndroid::activity();
         case ThreadedOpenGL: return !needsBasicRenderloopWorkaround() && QtAndroid::activity();
         case RasterGLSurface: return QtAndroid::activity();
+        case TopStackedNativeChildWindows: return false;
         default:
             return QPlatformIntegration::hasCapability(cap);
     }

src/widgets/kernel/qwindowcontainer.cpp

@@ -329,6 +329,19 @@ bool QWindowContainer::event(QEvent *e)
         e->accept();
         return true;
 #endif
+
+    case QEvent::Paint:
+    {
+        static bool needsPunch = !QGuiApplicationPrivate::platformIntegration()->hasCapability(
+            QPlatformIntegration::TopStackedNativeChildWindows);
+        if (needsPunch) {
+            QPainter p(this);
+            p.setCompositionMode(QPainter::CompositionMode_Source);
+            p.fillRect(rect(), Qt::transparent);
+        }
+        break;
+    }
+
     default:
         break;
     }