Add graphics/go-gift version 1.2.1

Package gift provides a set of useful image processing filters with no
external dependencies outside of the Go standard library.

SUPPORTED FILTERS:
Transformations
- Crop(rect image.Rectangle)
- CropToSize(width, height int, anchor Anchor)
- FlipHorizontal()
- FlipVertical()
- Resize(width, height int, resampling Resampling)
- ResizeToFill(width, height int, resampling Resampling, anchor Anchor)
- ResizeToFit(width, height int, resampling Resampling)
- Rotate(angle float32, backgroundColor color.Color, interpolation
  Interpolation)
- Rotate180()
- Rotate270()
- Rotate90()
- Transpose()
- Transverse()
Adjustments & effects
- Brightness(percentage float32)
- ColorBalance(percentageRed, percentageGreen, percentageBlue float32)
- ColorFunc(fn func(r0, g0, b0, a0 float32) (r, g, b, a float32))
- Colorize(hue, saturation, percentage float32)
- ColorspaceLinearToSRGB()
- ColorspaceSRGBToLinear()
- Contrast(percentage float32)
- Convolution(kernel []float32, normalize, alpha, abs bool, delta float32)
- Gamma(gamma float32)
- GaussianBlur(sigma float32)
- Grayscale()
- Hue(shift float32)
- Invert()
- Maximum(ksize int, disk bool)
- Mean(ksize int, disk bool)
- Median(ksize int, disk bool)
- Minimum(ksize int, disk bool)
- Pixelate(size int)
- Saturation(percentage float32)
- Sepia(percentage float32)
- Sigmoid(midpoint, factor float32)
- Sobel()
- Threshold(percentage float32)
- UnsharpMask(sigma, amount, threshold float32)

(nikita)