WebCL validator

A few years ago the WebCL working group created a validator which ensured
that OpenCL kernels loaded into browsers couldn't access memory they
weren't supposed to be able to. The project was open-source and is
available here:

https://github.com/KhronosGroup/webcl-validator

There's some interesting information about the overall approach in the
design writeup:

https://github.com/KhronosGroup/webcl-validator/blob/master/DESIGN.txt

Here are some kernels before and after transformation by the validator:

http://wolfviking0.github.io/webcl-translator/

There's also a nice presentation on the validator with some examples,
including timing measurements:

http://learningwebcl.com/2013/11/webcl-kernel-validator-explained/
http://learningwebcl.com/wp-content/uploads/2013/11/WebCLMemoryProtection.pdf

I remembered the overhead incorrectly. The 10% number I mentioned on
yesterday's conference call was the cost of zero-initializing all buffers
in WebCL, which was a different project than this one. The validator has
significantly more overhead; kernels run on the order of 1.5x - 3x slower,
because of needing to range check all pointer accesses against multiple
memory regions.

Let's study and learn from this previous work. WebGPU should ideally
enforce constraints on its compute kernels to reduce the overhead of any
run-time checks that are needed.

-Ken