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Update golang.org/x/image dependency (improves BMP and TIFF support).

Fix golden tests on arm64, ppc64le, s390x.

• Overall performance improvements
• New function - AdjustSaturation
• More tests and benchmarks

• New decode option AutoOrientation, that sets the auto-orientation mode. If auto-orientation is enabled, the image will be transformed after decoding according to the EXIF orientation tag (if present). By default it's disabled.
• Blur performance optimizations.
• New FormatFromExtension helper function.

• Fix race condition and simplify the New function.
• Add convenience function FormatFromFilename.

• Nearest-neighbor downsampling improved.
• New encode option added - PNGCompressionLevel.
• Minor readme/documentation updates.

• Image processing refactoring: most functions are now faster and allocate less memory.
• More tests and benchmarks added.
• More encode options added: in addition to JPEGQuality there are now three GIF encode options (GIFNumColors, GIFQuantizer, GIFDrawer).