Fix various UTF8 parsing bugs.

In particular, parse bad UTF8 by replacing the broken bits with U+FFFD
and resychronise correctly after broken sequences.

jv_unicode.h

@@ -2,7 +2,7 @@
 #define JV_UNICODE_H
 
 const char* jvp_utf8_next(const char* in, const char* end, int* codepoint);
-
+int jvp_utf8_is_valid(const char* in, const char* end);
 
 int jvp_utf8_decode_length(char startchar);