In ddvt source box from oldBox.
Enable clang with CC=clang.
Implement phConstraint* functions.
Implement phworld.
Make phClean static. Add phStaticContains.
Add phTestReset. Optimize phTest with phStaticContains.
Optimize _phDdvtUpdateLeaf.

Use intersect tests instead of checking if it is already stored.
Comment phDdvtPairNext.
Make DdvtPair methods public.
Experimental phDdvtUpdateChild optimization.
Add OpenGL/SDL non-interactive example.
Merge branch 'world'