fix headers to utilize include guards, else all hell breaks lose easily

when someone tries to use them.

cherry picked because github merge is a fucking retard
(cherry picked from commit 76e0b93999b38de024631d5c60b2c1b65f33a81f)
Merge branch 'sreich-add-header-guards-v2' of git://github.com/sreich/Chipmunk-Physics into sreich-sreich-add-header-guards-v2
Push/pop render state to support drawing during updates better
Move the frand_unit_circle() function somewhere more useful.
Applying a patch to fix deep circle/poly collisions
Refactoring to take out reliance on dumb C hacks
Fiddling with render settings to make AA lines thinner.