Make --features rand=1 available to the user

Use rand=255 to mean "randomize".

Part of https://github.com/harfbuzz/harfbuzz/pull/803

src/hb-ot-map.cc

diff --git a/src/hb-ot-map.cc b/src/hb-ot-map.cc
index 8617f05..cb70583 100644
--- a/src/hb-ot-map.cc
+++ b/src/hb-ot-map.cc
@@ -210,8 +210,8 @@
       /* Uses the global bit */
       bits_needed = 0;
     else
-      /* Limit to 8 bits per feature. */
-      bits_needed = MIN(8u, hb_bit_storage (info->max_value));
+      /* Limit bits per feature. */
+      bits_needed = MIN(HB_OT_MAP_MAX_BITS, hb_bit_storage (info->max_value));
 
     if (!info->max_value || next_bit + bits_needed > 8 * sizeof (hb_mask_t))
       continue; /* Feature disabled, or not enough bits. */