[buffer] Add API for random state

Fixes https://github.com/harfbuzz/harfbuzz/issues/4620

New API:
+hb_buffer_set_random_state()
+hb_buffer_get_random_state()

docs/harfbuzz-sections.txt

@@ -83,6 +83,8 @@ hb_buffer_set_not_found_glyph
 hb_buffer_get_not_found_glyph
 hb_buffer_set_replacement_codepoint
 hb_buffer_get_replacement_codepoint
+hb_buffer_set_random_state
+hb_buffer_get_random_state
 hb_buffer_normalize_glyphs
 hb_buffer_reverse
 hb_buffer_reverse_range

src/hb-buffer.cc

@@ -309,6 +309,7 @@ hb_buffer_t::clear ()
 
   deallocate_var_all ();
   serial = 0;
+  random_state = 1;
   scratch_flags = HB_BUFFER_SCRATCH_FLAG_DEFAULT;
 }
 
@@ -1359,6 +1360,49 @@ hb_buffer_get_not_found_glyph (const hb_buffer_t *buffer)
   return buffer->not_found;
 }
 
+/**
+ * hb_buffer_set_random_state:
+ * @buffer: An #hb_buffer_t
+ * @state: the new random state
+ *
+ * Sets the random state of the buffer. The state changes
+ * every time a glyph uses randomness (eg. the `rand`
+ * OpenType feature). This function together with
+ * hb_buffer_get_random_state() allow for transferring
+ * the current random state to a subsequent buffer, to
+ * get better randomness distribution.
+ *
+ * Defaults to 1 and when buffer contents are cleared.
+ * A value of 0 disables randomness during shaping.
+ *
+ * XSince: REPLACEME
+ **/
+void
+hb_buffer_set_random_state (hb_buffer_t    *buffer,
+			    unsigned        state)
+{
+  if (unlikely (hb_object_is_immutable (buffer)))
+    return;
+
+  buffer->random_state = state;
+}
+
+/**
+ * hb_buffer_get_random_state:
+ * @buffer: An #hb_buffer_t
+ *
+ * See hb_buffer_set_random_state().
+ *
+ * Return value:
+ * The @buffer random state
+ *
+ * XSince: REPLACEME
+ **/
+unsigned
+hb_buffer_get_random_state (const hb_buffer_t *buffer)
+{
+  return buffer->random_state;
+}
 
 /**
  * hb_buffer_clear_contents:

src/hb-buffer.h

@@ -487,6 +487,12 @@ hb_buffer_set_not_found_glyph (hb_buffer_t    *buffer,
 HB_EXTERN hb_codepoint_t
 hb_buffer_get_not_found_glyph (const hb_buffer_t *buffer);
 
+HB_EXTERN void
+hb_buffer_set_random_state (hb_buffer_t    *buffer,
+			    unsigned        state);
+
+HB_EXTERN unsigned
+hb_buffer_get_random_state (const hb_buffer_t *buffer);
 
 /*
  * Content API.

src/hb-buffer.hh

@@ -116,6 +116,7 @@ struct hb_buffer_t
 
   uint8_t allocated_var_bits;
   uint8_t serial;
+  uint32_t random_state;
   hb_buffer_scratch_flags_t scratch_flags; /* Have space-fallback, etc. */
   unsigned int max_len; /* Maximum allowed len. */
   int max_ops; /* Maximum allowed operations. */

src/hb-ot-layout-gsubgpos.hh

@@ -723,7 +723,6 @@ struct hb_ot_apply_context_t :
   bool auto_zwj = true;
   bool per_syllable = false;
   bool random = false;
-  uint32_t random_state = 1;
   unsigned new_syllables = (unsigned) -1;
 
   signed last_base = -1; // GPOS uses
@@ -788,8 +787,8 @@ struct hb_ot_apply_context_t :
   uint32_t random_number ()
   {
     /* http://www.cplusplus.com/reference/random/minstd_rand/ */
-    random_state = random_state * 48271 % 2147483647;
-    return random_state;
+    buffer->random_state = buffer->random_state * 48271 % 2147483647;
+    return buffer->random_state;
   }
 
   bool match_properties_mark (hb_codepoint_t  glyph,