Add 8-tap generic convolver

This commit introduces a new convolution function which will be used to
replace the existing subpixel interpolation functions. It is much the
same as the existing functions, but allows for changing the filter
kernel on a per-pixel basis, and doesn't bake in knowledge of the
filter to be applied or the size of the resulting block into the
function name.

Replacing the existing subpel filters will come in a later commit.

Change-Id: Ic9a5615f2f456cb77f96741856fc650d6d78bb91