Filling a developer's request by adding a cheap sharpen ported from Imblib2.

I evaluated this before and didn't like it because it's not as good as the convolve based one with it's fancy kernel and proper edge handling. But it's simple and faster. I also added it to the test app to make sure it works.

If your a developer handle it the same way as blur: use sharpen() for quick sharpening and use gaussianSharpen() for higher quality results.