Create common base classes for buffered streams

Most streams are backed by a memory buffer. Create common base classes
for this functionality to avoid code duplication.

common/rdr/BufferedInStream.cxx

@@ -0,0 +1,72 @@
+/* Copyright (C) 2002-2005 RealVNC Ltd.  All Rights Reserved.
+ * Copyright 2020 Pierre Ossman for Cendio AB
+ * 
+ * This is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ * 
+ * This software is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this software; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307,
+ * USA.
+ */
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <rdr/BufferedInStream.h>
+#include <rdr/Exception.h>
+
+using namespace rdr;
+
+static const size_t DEFAULT_BUF_SIZE = 8192;
+
+BufferedInStream::BufferedInStream(size_t bufSize_)
+  : bufSize(bufSize_ ? bufSize_ : DEFAULT_BUF_SIZE), offset(0)
+{
+  ptr = end = start = new U8[bufSize];
+}
+
+BufferedInStream::~BufferedInStream()
+{
+  delete [] start;
+}
+
+size_t BufferedInStream::pos()
+{
+  return offset + ptr - start;
+}
+
+size_t BufferedInStream::overrun(size_t itemSize, size_t nItems, bool wait)
+{
+  if (itemSize > bufSize)
+    throw Exception("BufferedInStream overrun: "
+                    "requested size of %lu bytes exceeds maximum of %lu bytes",
+                    (long unsigned)itemSize, (long unsigned)bufSize);
+
+  if (end - ptr != 0)
+    memmove(start, ptr, end - ptr);
+
+  offset += ptr - start;
+  end -= ptr - start;
+  ptr = start;
+
+  while (avail() < itemSize) {
+    if (!fillBuffer(start + bufSize - end, wait))
+      return 0;
+  }
+
+  size_t nAvail;
+  nAvail = avail() / itemSize;
+  if (nAvail < nItems)
+    return nAvail;
+
+  return nItems;
+}