Fix SDL canvas colors. Closes #1287.

The hello_world_sdl.cpp test seemed to be constructed wrongly; the old code actually produced a line in the box when run through native clang. This PR fixes that. I also added an `extern "C"` specifier, which seems necessary to get the test to compile natively on OS X.

I also discovered that a blank screen was being colored entirely red, instead of black. This has been fixed and a test added.