Allow using non-ASCII format strings in Printf.

src/solvespace.h

@@ -275,6 +275,7 @@ public:
     utf8_iterator& operator++()    { **this; p=n; n=NULL; return *this; }
     utf8_iterator  operator++(int) { utf8_iterator t(*this); operator++(); return t; }
     char32_t       operator*();
+    const char*    ptr() const { return p; }
 };
 class ReadUTF8 {
     const std::string &str;

src/textwin.cpp

@@ -445,7 +445,9 @@ void TextWindow::Printf(bool halfLine, const char *fmt, ...) {
             }
         }
 
-        fmt++;
+        utf8_iterator it(fmt);
+        it++;
+        fmt = it.ptr();
     }
     while(c < MAX_COLS) {
         meta[r][c].fg = fg;