Switch all the constant strings in graphics code to use the new
variables.
This avoids the cost of instantiating CStrInterns at runtime every
frame.
This was SVN commit r13906.
the old debug_assert always ran and tested the expression, which slows
down release builds. wrapping them in #ifndef NDEBUG is clumsy. the new
ASSERT behaves like assert and ENSURE like the old debug_assert. Let's
change any time-critical but not-super-important ENSURE to ASSERT to
speed up release builds. (already done in bits.h and unique_range.h)
This was SVN commit r9362.
