New dedicated horizontal blur function

SO MUCH FASTER.  Now that I think about it, I could probably speed up my
box blur by using this new function to horizontally blur an image,
rotating the image 90 degrees, then applying a second horizontal blur.
Might have to test this in the future.
Tie radial blur into the new horizontal blur function

My my my, this is much better.  Because the horizontal blur function
supports variable left/right offsets, I added a "symmetry" option to the
radial blur tool.  Checking it blurs equally in both +/- angle
directions, while unchecking it only apples in the + direction.