Clean up internal clipboard handling

We now filter incoming data, which means we can start assuming the
clipboard data is always null terminated. This allows us to clean
up a lot of the internal handling.
diff --git a/tests/decperf.cxx b/tests/decperf.cxx
index 301e45e..df5214f 100644
--- a/tests/decperf.cxx
+++ b/tests/decperf.cxx
@@ -54,7 +54,7 @@
   virtual void framebufferUpdateEnd();
   virtual void setColourMapEntries(int, int, rdr::U16*);
   virtual void bell();
-  virtual void serverCutText(const char*, rdr::U32);
+  virtual void serverCutText(const char*);
 
 public:
   double cpuTime;
@@ -122,7 +122,7 @@
 {
 }
 
-void CConn::serverCutText(const char*, rdr::U32)
+void CConn::serverCutText(const char*)
 {
 }