Implemented text and clipping primitives in the frontend, and added

two new simple games `fifteen' and `sixteen'.

[originally from svn r4173]

misc.c

@@ -0,0 +1,25 @@
+/*
+ * misc.c: Miscellaneous helpful functions.
+ */
+
+#include <assert.h>
+#include <stdlib.h>
+
+#include "puzzles.h"
+
+int rand_upto(int limit)
+{
+    unsigned long divisor = RAND_MAX / (unsigned)limit;
+    unsigned long max = divisor * (unsigned)limit;
+    unsigned long n;
+
+    assert(limit > 0);
+
+    do {
+        n = rand();
+    } while (n >= max);
+
+    n /= divisor;
+
+    return (int)n;
+}