wasm: always call destroy() before deleting a screen

QWasmCompositor currently has the requirement that destroying it
requires freeing a GL texture, which requires a valid GL context,
which requires a valid screen, in order to get to the native context
on the canvas. 

For this reason QWasmScreen has a destroy() function which is called
before deleting the QScreen and QPlatformScreen.

Make sure we call destroy() also when deleting all screens in the
QWasmIntegration destructor. Move the common logic into a new
deleteScreen() function which replaces destroy().

Change-Id: I628f13c868808db539effff9b29ecbefac23abc9
Reviewed-by: Aleksandr Reviakin <aleksandr.reviakin@qt.io>
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>
Reviewed-by: David Skoland <david.skoland@qt.io>

src/plugins/platforms/wasm/qwasmintegration.cpp

@@ -196,7 +196,8 @@ QWasmIntegration::~QWasmIntegration()
     delete m_drag;
 
     for (const auto &elementAndScreen : m_screens)
-        QWindowSystemInterface::handleScreenRemoved(elementAndScreen.second);
+        elementAndScreen.second->deleteScreen();
+
     m_screens.clear();
 
     s_instance = nullptr;
@@ -338,10 +339,7 @@ void QWasmIntegration::removeScreen(const emscripten::val &element)
         qWarning() << "Attempting to remove non-existing screen for element" << QWasmString::toQString(element["id"]);;
         return;
     }
-    QWasmScreen *exScreen = it->second;
-    m_screens.erase(it);
-    exScreen->destroy(); // clean up before deleting the screen
-    QWindowSystemInterface::handleScreenRemoved(exScreen);
+    it->second->deleteScreen();
 }
 
 void QWasmIntegration::resizeScreen(const emscripten::val &element)

src/plugins/platforms/wasm/qwasmscreen.cpp

@@ -131,9 +131,10 @@ QWasmScreen::~QWasmScreen()
     m_canvas.set(m_canvasResizeObserverCallbackContextPropertyName, emscripten::val(intptr_t(0)));
 }
 
-void QWasmScreen::destroy()
+void QWasmScreen::deleteScreen()
 {
     m_compositor->destroy();
+    QWindowSystemInterface::handleScreenRemoved(this);
 }
 
 QWasmScreen *QWasmScreen::get(QPlatformScreen *screen)

src/plugins/platforms/wasm/qwasmscreen.h

@@ -55,7 +55,7 @@ class QWasmScreen : public QObject, public QPlatformScreen
 public:
     QWasmScreen(const emscripten::val &containerOrCanvas);
     ~QWasmScreen();
-    void destroy();
+    void deleteScreen();
 
     static QWasmScreen *get(QPlatformScreen *screen);
     static QWasmScreen *get(QScreen *screen);