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.
Hm... the browser does the right thing here, but I'd probably feel more comfortable just writing it out as 65535.

Also: Worst. Data Locality. EVER! :D