Revert "buffer: remove unused _drain method"

This reverts commit b3f42f9.

include/sqsh_primitive_private.h

@@ -137,6 +137,19 @@ SQSH_NO_EXPORT SQSH_NO_UNUSED int sqsh__buffer_append(
 SQSH_NO_EXPORT SQSH_NO_UNUSED int
 sqsh__buffer_move(struct SqshBuffer *buffer, struct SqshBuffer *source);
 
+/**
+ * @internal
+ * @memberof SqshBuffer
+ * @brief resets the buffer size to 0.
+ *
+ * This does not free the memory allocated by the buffer so that
+ * the buffer can be reused.
+ *
+ * @param[in,out] buffer The SqshBuffer to drain.
+ */
+
+void sqsh__buffer_drain(struct SqshBuffer *buffer);
+
 /**
  * @internal
  * @memberof SqshBuffer

lib/primitive/buffer.c

@@ -114,6 +114,11 @@ sqsh__buffer_move(struct SqshBuffer *buffer, struct SqshBuffer *source) {
 	return 0;
 }
 
+void
+sqsh__buffer_drain(struct SqshBuffer *buffer) {
+	buffer->size = 0;
+}
+
 const uint8_t *
 sqsh__buffer_data(const struct SqshBuffer *buffer) {
 	return buffer->data;