simplify calculations by using intermediate results

instead of calculating the same stuff multiple times

src/ImageSharp/ColorSpaces/Conversion/Implementation/Converters/CieXyzToHunterLabConverter.cs

@@ -45,9 +45,11 @@ namespace SixLabors.ImageSharp.ColorSpaces.Conversion.Implementation
             float ka = ComputeKa(this.HunterLabWhitePoint);
             float kb = ComputeKb(this.HunterLabWhitePoint);
 
-            float l = 100 * MathF.Sqrt(y / yn);
-            float a = ka * (((x / xn) - (y / yn)) / MathF.Sqrt(y / yn));
-            float b = kb * (((y / yn) - (z / zn)) / MathF.Sqrt(y / yn));
+            float yByYn = y / yn;
+            float sqrtYbyYn = MathF.Sqrt(yByYn);
+            float l = 100 * sqrtYbyYn;
+            float a = ka * (((x / xn) - yByYn) / sqrtYbyYn);
+            float b = kb * ((yByYn - (z / zn)) / sqrtYbyYn);
 
             if (float.IsNaN(a))
             {

src/ImageSharp/ColorSpaces/Conversion/Implementation/Converters/HunterLabToCieXyzConverter.cs

@@ -26,9 +26,12 @@ namespace SixLabors.ImageSharp.ColorSpaces.Conversion.Implementation
             float ka = ComputeKa(input.WhitePoint);
             float kb = ComputeKb(input.WhitePoint);
 
-            float y = ImageMaths.Pow2(l / 100F) * yn;
-            float x = (((a / ka) * MathF.Sqrt(y / yn)) + (y / yn)) * xn;
-            float z = (((b / kb) * MathF.Sqrt(y / yn)) - (y / yn)) * (-zn);
+            float pow = ImageMaths.Pow2(l / 100F);
+            float sqrtPow = MathF.Sqrt(pow);
+            float y = pow * yn;
+
+            float x = (((a / ka) * sqrtPow) + pow) * xn;
+            float z = (((b / kb) * sqrtPow) - pow) * (-zn);
 
             return new CieXyz(x, y, z);
         }

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -896,9 +896,9 @@ namespace SixLabors.ImageSharp.Formats.Png
             ref byte sourceRef = ref MemoryMarshal.GetReference(source);
             ref byte resultRef = ref MemoryMarshal.GetReference(result);
 
-            byte mask = (byte)(0xFF >> (8 - bits));
-            byte shift0 = (byte)(8 - bits);
             int shift = 8 - bits;
+            byte mask = (byte)(0xFF >> shift);
+            byte shift0 = (byte)shift;
             int v = 0;
             int resultOffset = 0;
 

src/ImageSharp/Processing/Processors/Quantization/FrameQuantizerBase{TPixel}.cs

@@ -139,8 +139,10 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         protected byte GetTransparentIndex()
         {
             // Transparent pixels are much more likely to be found at the end of a palette.
-            int index = this.paletteVector.Length - 1;
-            for (int i = this.paletteVector.Length - 1; i >= 0; i--)
+            int paletteVectorLengthMinus1 = this.paletteVector.Length - 1;
+
+            int index = paletteVectorLengthMinus1;
+            for (int i = paletteVectorLengthMinus1; i >= 0; i--)
             {
                 ref Vector4 candidate = ref this.paletteVector[i];
                 if (candidate.Equals(default))