Evas filters: Prepare optimization paths for BOX blur

Evas filters: Prepare optimization paths for BOX blur

Actually, there is a very nice trick with BOX blur.
Pass BOX blur 3 times and you can approximate a GAUSSIAN
blur with up to 3% accuracy. This is way more than enough
for just a simple graphical effect.

So, despite the crappy quality of BOX blur, we should
optimize it a lot so we can replace large GAUSSIAN blurs
with series of BOX blurs instead.

Source: Wikipedia's page on box blur :)

This commit also moves around some duplicated definitions.