Fixed the Unsharp Mask filter

Now it has the following features:
1) Uses Gaussian for the initial blurring instead of home-grown
circle mask
2) Supports floating point Radius value
3) Fixed the bug with wrong channelFlags are feed to the blur algo.
They didn't contain alpha channel before, resulting in really weird
results generated.
4) Uses Luminocity masking instead of mixing colors directly in RGB