Building VTK-9 with Emscripten+GLES for rendering

Hey Brad,

My understanding (which might be wrong) is that as soon as I provide a GLES library, I don’t necessary need the OpenGL one (probably only true for iOS and Emscripten builds). It means that OpenGL is given to me by the GLES one while having a more limited API (The GL code should have some ifdef with GLES so slightly different path could be taken inside the GL code).

@ken-martin might be able to provide more context on that aspect.

Thanks Brad for looking into that,

Seb