Generate an error in WebGL 2 if drawing with only instanced attribs

INVALID_OPERATION is generated if drawing with only instanced attribs or
if drawing without any attribs at all.

This makes creating a Direct3D-backed implementation less complicated,
since in Direct3D drawing always requires non-instanced vertex data.

The restrictions are a slightly tighter version of what is in the
ANGLE_instanced_arrays specification.