Add simple benchmark for drawElements index validation overhead

The test changes data in an index buffer and calls drawElements on it.
There are always some invalid indices in the buffer, so that any drawing
is not performed but the test just measures the overhead from uploading
data and performing validation.

The test is written to specifically target implementations in Chrome and
Firefox in March 2014, so some of the results might be slightly
misleading on other implementations.
Remove problematic drawElements benchmark case

The removed test case was designed specifically to hit the worst case of
the algorithm in Firefox, and if the intention was misinterpreted it
could put on undue pressure to optimize it at the cost of real world
performance.
Restore drawElements benchmark hitting the worst case in Firefox

It was decided that the benchmark should be included after all as long as
it is clearly marked as not based on real-world content.
Add missing header to drawElements validation test
Merge pull request #501 from NVIDIA/drawelements-bench

Add simple benchmark for drawElements index validation overhead