If the buffer is smaller than the blur kernel, then artifacts appear
and CRASHES happen because we read/write out of the buffer bounds.
Output buffer must be larger than the kernel diameter.
Input buffer's size is used to reduce the blurring effect on the edges.
Currently supported:
- Box blur for Alpha and RGBA
- Gaussian blur for Alpha and RGBA
Motion blur is not implemented.
These effects are SLOW. Gaussian blur is based on a true gaussian
curve for small radii, or a sine-based approximation.
The true gaussian might need to be removed for consistency, since
it gives slightly different results from the sine one (less blur).
It is not possible to mix Alpha and RGBA surfaces in this filter.
Jean-Philippe Andre