Evas filters: Optimize alpha box blur

Evas filters: Optimize alpha box blur

Use two optimizable functions for BOX blur: vertical and horizontal.
These functions will run as many times as requested (from 1 to 6 max).

The horizontal case is pretty straightforward as the source is already
contiguous (nice in terms of cache hits). The only catch is to swap
src and dst without ever writing to the input buffer.

In case of vertical blur, we apply the same method as above, after
rotating the column into a horizontal (contiguous) span, and rotating
it back afterwards.

Now, the same needs to be done for RGBA :)