Combine all of the prior_* static vars to a single struct.
Combine all of the prior_* static vars to a single struct.

Reduces the cascading if/else-if chain to a single memcmp(), and simplifies
copying the new settings as well.

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