Simplify get_surface_impl()
Simplify get_surface_impl()

Implement the SIMD versions in terms of the non-SIMD versions
TODO: see if we can spare even more overloads

szellmann/visionaray,ukoeln-vis/ctpperf,tu500/visionaray,tu500/visionaray,ukoeln-vis/ctpperf,szellmann/visionaray