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.
Added test for -1 index.

Some implementations may treat a -1 in the element array buffer
(65535 for 16-bit indices) as a primitive restart marker.
Instead for ES2 it should always be a valid regular index.
Merge pull request #642 from c0d1f1ed/negative-one-index

Added test for -1 index.
Merge pull request #640 from NVIDIA/all-attribs-instanced

Generate an error in WebGL 2 if drawing with only instanced attribs
Add test for drawing without any enabled attributes

This corner case is not very useful but still correct according to the
spec.
Add stress test version of point-no-attributes