Consider adding a no-opengl raster backend for Qt on WebAssembly

It is unlikely that we will see a browser that supports WASM but not WebGL, at least for the near future. However, some browsers maintain GPU blacklists, which means that there in practice will be systems with wasm but no OpenGL support. One example of this are 2011-erea macBooks with the Intel HD3000 GPU, which are blacklisted on Chrome.

We can't support OpenGL on such systems, but we can support raster windows and Qt Quick on the raster backend, and fall back transparently.