Add unsharp mask

[puzrin]

For practical tasks, it's required to combine downscale with unsharp mask.

1. For convenience, it worth to use the same prams as photoshop USM http://stackoverflow.com/questions/13296645/
2. More to read http://stackoverflow.com/questions/2938162/
3. Gimp implementation https://git.gnome.org/browse/gimp/tree/plug-ins/common/unsharp-mask.c
4. Luminance / brightness math http://stackoverflow.com/questions/596216/
5. Bugzilla discusion https://bugzilla.mozilla.org/show_bug.cgi?id=867594

Details:

• It's ok to support only 1px blur radius. That will require minimal convolution matrix 3x3, and will be fast enougth
• (!) unsharp luminance, not separate channels, to avoid color shift
• (!) use high precision (> 8 bits) for luminance calculations.

Algorythm:

1. build array of L (luminance) values (16 or 32 bits). 4x less points for convolution, as bonus.
2. for each L point, calculate convolution, check threshold and update rgb on original if needed.
3. additional requirements:
   • use integer math only (int32)
   • don't use division

Blur:

• http://elynxsdk.free.fr/ext-docs/Blur/Fast_box_blur.pdf
• http://www.csse.uwa.edu.au/~pk/research/pkpapers/FastGaussianSmoothing.pdf
• http://web.stanford.edu/class/cs448f/lectures/2.2/Fast%20Filtering.pdf

Other:

• (!) https://software.intel.com/en-us/articles/iir-gaussian-blur-filter-implementation-using-intel-advanced-vector-extensions
• http://web.stanford.edu/class/cs448f/lectures/3.1/Fast%20Filtering%20Continued.pdf

[puzrin]

Implementation details:

• Do all transforms on Luminance channel of HSL (RGB -> L -> unsharp -> RGB)
• Blur type - Box, 2 times.
• Integer math only (int32 max).
• No division on hot path.
• Optimize calculations
• Add benchmark (blur + color corrections)

[puzrin]

Solved by 60c6051

Will be closed after some polish & docs update.