Used mapped memory for setting loop_mem->offsets_mem.
Used mapped memory for setting loop_mem->offsets_mem.

This is enabled by #define MAP_OFFSETS [01], and can almost double performance
when enabled.

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