Add some GLES3 functions for WebGL2.

Just noticed that this is missing `glTexImage3D`, and then noticed https://www.khronos.org/registry/webgl/specs/latest/2.0/#5.14 . Commented https://github.com/KhronosGroup/WebGL/issues/542#issuecomment-58999118 on that, I don't think what's happening here is appropriate.