[API] Rename hb_buffer_ensure() to hb_buffer_pre_allocate()

The new name is self-documenting.

src/hb-buffer.cc

@@ -255,7 +255,7 @@ hb_buffer_reset (hb_buffer_t *buffer)
 }
 
 hb_bool_t
-hb_buffer_ensure (hb_buffer_t *buffer, unsigned int size)
+hb_buffer_pre_allocate (hb_buffer_t *buffer, unsigned int size)
 {
   return _hb_buffer_ensure (buffer, size);
 }
@@ -453,7 +453,7 @@ hb_bool_t
 hb_buffer_set_length (hb_buffer_t  *buffer,
 		      unsigned int  length)
 {
-  if (!hb_buffer_ensure (buffer, length))
+  if (!_hb_buffer_ensure (buffer, length))
     return FALSE;
 
   /* Wipe the new space */

src/hb-buffer.h

@@ -107,8 +107,8 @@ void
 hb_buffer_reset (hb_buffer_t *buffer);
 
 hb_bool_t
-hb_buffer_ensure (hb_buffer_t  *buffer,
-		  unsigned int  size);
+hb_buffer_pre_allocate (hb_buffer_t  *buffer,
+		        unsigned int  size);
 
 void
 hb_buffer_reverse (hb_buffer_t *buffer);