fix use of stbrp_coord if no stb_rect_pack;

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

tests/stretch_test.c

@@ -1,3 +1,7 @@
+// check that stb_truetype compiles with no stb_rect_pack.h
+#define STB_TRUETYPE_IMPLEMENTATION
+#include "stb_truetype.h"
+
 #include "stretchy_buffer.h"
 #include <assert.h>