Minor bitmap optimizations

Former-commit-id: b82f0fb8a821c2be2f24f602076938999a99c98f
Former-commit-id: c3771878dc9096ee355c170129765ec6b0f488f2
Former-commit-id: 49cc7337fcab45f5957dfea0b650f6244f57f0fa

src/ImageProcessor/Formats/Bmp/BmpDecoderCore.cs

@@ -87,7 +87,8 @@ namespace ImageProcessor.Formats
 
                 if (colorMapSize > 0)
                 {
-                    if (colorMapSize > 255 * 4)
+                    // 255 * 4
+                    if (colorMapSize > 1020)
                     {
                         throw new ImageFormatException($"Invalid bmp colormap size '{colorMapSize}'");
                     }
@@ -242,8 +243,8 @@ namespace ImageProcessor.Formats
         private void ReadRgb16(float[] imageData, int width, int height)
         {
             // We divide here as we will store the colors in our floating point format.
-            const int ScaleR = (256 / 32) / 32;
-            const int ScaleG = (256 / 64) / 64;
+            const float ScaleR = 0.25F; // (256 / 32) / 32
+            const float ScaleG = 0.0625F; // (256 / 64) / 64
 
             int alignment;
             byte[] data = this.GetImageArray(width, height, 2, out alignment);