Added a few more configurable options in filter_c.c.
Added a few more configurable options in filter_c.c.

So far, just turning on REGISTER_FILTER is fastest. Other options
are a bit slower, but if I can restructure the filters to accept
just source/output pointers and calculate an offset into each, then
I can try out performance in Cuda/OpenCL.

awatry/libvpx.opencl,awatry/libvpx.opencl,awatry/libvpx.opencl,awatry/libvpx.opencl