benchmark.py: Add Halide cases.
benchmark: Add a 4K image Halide benchmark. The sample is
in public domain:

http://www.uhdwallpapers.org/2014/11/garden-summer-chalet-nature-landscape.html
Disable the custom bufalloc by default for pthread for now.

Halide cases had memory allocation explosions due to it, need to
investigate why.
Fix an issue where pthread malloc allocator freed host
pointers.