Added Generic Convolution Filter to NVidia Texture Tools

I've recently added generic convolution filter to the NVidia Textures Tools version I'm using at work. (This is based upon unreleased version that I grabbed from NVTT Google Code page. I wanted to contribute it back to the code base or share with the public, so I asked for the permission from the work, which I'm still waiting for.

At the end it's not hard. If you just dig around the source code, you will find a way to implement it in less than an hour. Or, just wait until I get the OK sign from my work :)