wasm: fix setting translucent background

We need to set the requested SurfaceFormat which may contain
the alphaBufferSize in order for
the GL context to have the alpha attribute

Fixes: QTBUG-77303
Change-Id: I39860e24de49a255ab7e73bca78af92e6c074d0d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/wasm/qwasmcompositor.cpp

@@ -694,7 +694,7 @@ void QWasmCompositor::frame()
 
     if (m_context.isNull()) {
         m_context.reset(new QOpenGLContext());
-        //mContext->setFormat(mScreen->format());
+        m_context->setFormat(someWindow->window()->requestedFormat());
         m_context->setScreen(screen()->screen());
         m_context->create();
     }