QOffScreenSurface::isValid always returns false on WebAssembly

If you call QOffScreenSurface::isValid on a WebAssembly build it always returns false. This is because it is implemented via QWasmOffscreenSurface which provides no ::isValid implementation, so the default super implementation in QPlaformOffscreenSurface is called instead, and this unconditionally returns false.

I guess a sensible implementation might be to test m_offscreenCanvas against emscripten::val::undefined(), but that's speculative on my part.