Visible artifacts in imageBlur

[jereroy]

There are visible artifacts in the image blur sample. With 10 iterations and a filter size of 2, they are particularly obvious :

Artifacts are only visible when the filter dimensions are even. By default, the filter is odd, but dat.gui rounds to an even dimension when using the slider. Also, using only odd dimensions, an offset is visible when increasing the number of iterations atm. The character "moves" to the bottom right of the screen.