Feature: adaptive histogram equalization (SixLabors#673)

* first version of sliding window adaptive histogram equalization

* going now from top to bottom of the image, added more comments

* using memory allocator to create the histogram and the cdf

* mirroring rows which exceeds the borders

* mirroring also left and right borders

* gridsize and cliplimit are now parameters of the constructor

* using Parallel.For

* only applying clipping once, effect applying it multiple times is neglectable

* added abstract base class for histogram equalization, added option to enable / disable clipping

* small improvements

* clipLimit now in percent of the total number of pixels in the grid

* optimization: only calculating the cdf until the maximum histogram index

* fix: using configuration from the parameter instead of the default

* removed unnecessary loops in CalculateCdf, fixed typo in method name AddPixelsToHistogram

* added different approach for ahe: image is split up in tiles, cdf is computed for each tile. Grey value will be determined by interpolating between 4 tiles

* simplified interpolation between the tiles

* number of tiles is now fixed and depended on the width and height of the image

* moved calculating LUT's into separate method

* number of tiles is now part of the options and will be used with the sliding window approach also, so both methods are comparable

* removed no longer valid xml comment

* attempt fixing the borders

* refactoring to improve readability

* linear interpolation in the border tiles

* refactored processing the borders into separate methods

* fixing corner tiles

* fixed build errors

* fixing mistake during merge from upstream: setting test images to "update Resize reference output because of improved ResizeKernelMap calculations"

* using Parallel.ForEach for all inner tile calculations

* using Parallel.ForEach to calculate the lookup tables

* re-using pre allocated pixel row in GetPixelRow

* fixed issue with the border tiles, when tile width != tile height

* changed default value for ClipHistogram to false again

* alpha channel from the original image is now preserved

* added unit tests for adaptive histogram equalization

* Update External

* 2x faster adaptive tiled processor

* Remove double indexing and bounds checks

* Begin optimizing the global histogram

* Parallelize GlobalHistogramEqualizationProcessor

* Moving sliding window from left to right instead of from top to bottom

* The tile width and height is again depended on the image width: image.Width / Tiles

* Removed keeping track of the maximum histogram position

* Updated reference image for sliding window AHE for moving the sliding window from left to right

* Removed unnecessary call to Span.Clear(), all values are overwritten anyway

* Revert "Moving sliding window from left to right instead of from top to bottom"

This reverts commit 8f19e5e.

# Conflicts:
#	src/ImageSharp/Processing/Processors/Normalization/AdaptiveHistEqualizationSWProcessor.cs

* Split GetPixelRow in two version: one which mirrors the edges (only needed in the borders of the images) and one which does not

* Refactoring and cleanup sliding window processor

* Added an upper limit of 100 tiles

* Performance tweaks

* Update External

Author: brianpopow

tests/ImageSharp.Tests/TestImages.cs

@@ -141,6 +141,7 @@ public static class Bad
                 public const string Testorig420 = "Jpg/baseline/testorig.jpg";
                 public const string MultiScanBaselineCMYK = "Jpg/baseline/MultiScanBaselineCMYK.jpg";
                 public const string Ratio1x1 = "Jpg/baseline/ratio-1x1.jpg";
+                public const string LowContrast = "Jpg/baseline/AsianCarvingLowContrast.jpg";
                 public const string Testorig12bit = "Jpg/baseline/testorig12.jpg";
                 public const string YcckSubsample1222 = "Jpg/baseline/ycck-subsample-1222.jpg";