move all extension methods under a subfolder

(non-namespace providing)

src/ImageSharp/ImageSharp.csproj.DotSettings

@@ -7,6 +7,7 @@
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=pixelformats_005Cpixelimplementations/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=pixelformats_005Cpixeltypes/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=pixelformats_005Cutils/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=processing_005Cextensions/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=processing_005Cprocessors_005Cconvolution_005Ckernels/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=processing_005Cprocessors_005Ctransforms_005Cresamplers/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=processing_005Cprocessors_005Ctransforms_005Cresize/@EntryIndexedValue">True</s:Boolean></wpf:ResourceDictionary>

src/ImageSharp/Processing/HistogramEqualizationExtension.cs → src/ImageSharp/Processing/Extensions/HistogramEqualizationExtensions.cs

@@ -8,7 +8,7 @@ namespace SixLabors.ImageSharp.Processing
     /// <summary>
     /// Defines extension that allow the adjustment of the contrast of an image via its histogram.
     /// </summary>
-    public static class HistogramEqualizationExtension
+    public static class HistogramEqualizationExtensions
     {
         /// <summary>
         /// Equalizes the histogram of an image to increases the contrast.