wasm: QSGContext::initialize: stencil buffer support missing, expect rendering errors

Testing the sgenging example I noticed an error 

"QSGContext::initialize: stencil buffer support missing, expect rendering errors"

Which led me to debug a bit.

The example explicitly requests a depthBufferSize of 16, which would mean we create a webgl context that has depthBuffer support, but since WebGL does not have support for separate depth and stencil buffers, we need to enable stencil buffer as well.