Fix box blur symmetry & simplify code

[rmn20]

Previous version of box blur kernel was asymmetrical, because the leftmost and topmost pixels were subtracted from the average values too early, causing the image to offset by GAUSSIAN_BLUR_ITERATIONS pixels after blurring. You can see this at the rightmost and bottommost borders of the image. This commit fixes this bug.

Original image	Old box blur	Fixed box blur

Image is also a bit more blurry now since the blur kernel is now 1 pixel wider.
PS. Perhaps GAUSSIAN_BLUR_ITERATIONS could be a function argument rather than a predefined value?

[raysan5]

@rmn20 Nice! Great improvement! Actually I didn't notice the issue with previous implementation, but not it looks way better! Thanks!

[rmn20]

@raysan5 You can see the offset issue with previous implementation much more obviously, if you apply box blur several times.
For example, here I've applied box blur with radius 1 three times:

Original image	Old box blur	Fixed box blur