Made spatially_filter_image() use SIMD instructions when filtering float data.

dlib/image_transforms/spatial_filtering_abstract.h

@@ -60,6 +60,9 @@ namespace dlib
             - #out_img.nr() == in_img.nr()
             - returns a rectangle which indicates what pixels in #out_img are considered 
               non-border pixels and therefore contain output from the filter.
+            - if (use_abs == false && all images and filers contain float types) then
+                - This function will use SIMD instructions and is particularly fast.  So if
+                  you can use this form of the function it can give a decent speed boost.
     !*/
 
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