fix a few assert()s that weren't STBTT_asserts();
fix missing cast for C++
fix typo in C++ test compilation that prevented it from trying to compile stb_truetype

   STBTT_POINT_SIZE
   documentation for above

stb_rect_pack:
   STBRP_ASSERT

oversample test tweaks

stb_truetype: oversampling, including oversampling demo app