Rewrite central convolution filter code

PD now provides a standalone "ConvolveDIB" function, which will apply an
arbitrary 5x5 convolution filter to any source and destination DIB.
This makes it much easier to utilize convolution filters internally,
without having to reproduce the full convolution filter function over
and over again.

Existing functions that make direct use of arbitrary convolution filters
have been rewritten to account for this update