Embedded constant image host_dirty flag initialized to true.
Constant flags for embedded image GlobalVariables unset for GPU hosts.
Merge pull request #229 from jrprice/embedded-image-ocl

Fix for embedded constant images for GPU targets
Slightly more aggressive in simplification of loop extents before unroll
Merge branch 'master' of github.com:halide/Halide
Fix build on OS X/clang