wasm: provide isValid implementation for QWasmOffscreenSurface

Fixes: QTBUG-120350
Pick-to: 6.6
Change-Id: I0cb2ba66a2d27f3d96a4a6a1cb85695f6b305178
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
(cherry picked from commit 139d57a7e906b84672024661a58628918ceff928)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/wasm/qwasmoffscreensurface.cpp

@@ -27,4 +27,9 @@ QWasmOffscreenSurface::~QWasmOffscreenSurface()
     emscripten::val::module_property("specialHTMLTargets").delete_(m_specialTargetId);
 }
 
+bool QWasmOffscreenSurface::isValid() const
+{
+    return !m_offscreenCanvas.isNull() && !m_offscreenCanvas.isUndefined();
+}
+
 QT_END_NAMESPACE

src/plugins/platforms/wasm/qwasmoffscreensurface.h

@@ -20,6 +20,7 @@ public:
     ~QWasmOffscreenSurface() final;
 
     const std::string &id() const { return m_specialTargetId; }
+    bool isValid() const override;
 
 private:
     std::string m_specialTargetId;