faux solarization functionality for images

Works by generating a histogram, smoothing it, and applying the result as a function on each pixel.

To compile: make

To run: ./solarize [-v] [-g] [-i] [-t THRESHOLD] [-w WINDOW] FILE...

• -v: Currently does nothing, should probably be verbosity control.
• -g: Converts color image to grayscale.
• -i: Invert the final image. Sometimes has better results? Play with it.
• -t THRESHOLD: Sets the threshold pixel at which the image's original value will be used (i.e. the curve will be bypassed for very black pixels). This can prevent large swathes of black in some images. Defaults to 0.
• -w WINDOW: Sets the size of the moving average window. A larger window results in less "rough" images (less starkly saturated). Somewhere around 20-40 seems to be the sweet spot.
• You can use multiple files. It will spawn a thread per image, hopefully taking advantage of parallelism.