Attempt to clarify behavior of linkProgram as it relates to useProgram.
Add a test for out-of-range constant indices failing compilation

ESSL 1.00 spec states that a constant index must generate a compilation
error if it is out of range. Test this with a variety of constant
expressions.
Move the proposed constant index out of range test to extras for now

This test requires large changes without that much real-world impact, so
it should not be a part of the initial WebGL 2.0 conformance suite.
Merge pull request #806 from NVIDIA/constant-index-out-of-range-test

Add a test for out-of-range constant indices failing compilation
Merge pull request #804 from kenrussell/clarify-program-behavior

Attempt to clarify behavior of linkProgram as it relates to useProgram.