Example applying a convolution kernel to an image

This example was basically to see how much image data manipulation could be done while still getting a high frame rate.

For those that have experimented with this kind of thing, I was surprised to find that working with `Uint32Array` views of the data was *not* universally faster.  In this particular example, I think the extra clamping operations are a hit (where we get this with no extra JS processing using the `Uint8ClampedArray` view).

If anybody has any additional ideas of faster convolution strategies, let me know.  See this [performance test](http://jsperf.com/canvas-convolution/4) for a comparison of a few Uint8/Uint32 read/write alternatives.