Sharpens pixels of a frame using the surrounding pixels. Function takes it a texture, a coordinate and the render size, and returns the pixel value for that coordinate after applying the sharpening convolution.
Adapted from https://igortrindade.wordpress.com/2010/04/23/fun-with-opengl-and-shaders/
sharpen(sampler2D inputTexture, vec2 coord, vec2 imageSize) => vec4
MIT. See LICENSE.md for details.