When redrawing screen, read the last written frame

This should fix the bug with loadstates, where wrong screenshot was
displayed in certain race condition.

include/library/triplebuffer.hpp

@@ -45,6 +45,12 @@ public:
  * Throws std::logic_error: If write count is 0.
  */
 	void put_write() throw(std::logic_error);
+/**
+ * Call specified function synchronously for last written buffer.
+ *
+ * The buffer number is passed to specified function.
+ */
+	void read_last_write_synchronous(std::function<void(unsigned)> fn) throw();
 private:
 	threads::lock lock;
 	unsigned last_complete;		//Number of last completed buffer
@@ -103,6 +109,15 @@ public:
  * Throws std::logic_error: If write count is 0.
  */
 	void put_write() throw(std::logic_error) { l.put_write(); }
+/**
+ * Call specified function synchronously for last written buffer.
+ *
+ * The buffer itself is passed to the function.
+ */
+	void read_last_write_synchronous(std::function<void(T&)> fn) throw()
+	{
+		l.read_last_write_synchronous([this,fn](unsigned x){ fn(*objs[x]); });
+	}
 private:
 	T* objs[3];
 	logic l;

src/core/framebuffer.cpp

@@ -170,9 +170,8 @@ void emu_framebuffer::redraw_framebuffer(framebuffer::raw& todraw, bool no_lua,
 
 void emu_framebuffer::redraw_framebuffer()
 {
-	render_info& ri = buffering.get_read();
-	framebuffer::raw copy = ri.fbuf;
-	buffering.put_read();
+	framebuffer::raw copy;
+	buffering.read_last_write_synchronous([&copy](render_info& ri) { copy = ri.fbuf; });
 	//Redraws are never spontaneous
 	redraw_framebuffer(copy, last_redraw_no_lua, false);
 }

src/library/triplebuffer.cpp

@@ -60,4 +60,11 @@ void logic::put_write() throw(std::logic_error)
 	if(!count_write)
 		last_complete = current_write;
 }
+
+void logic::read_last_write_synchronous(std::function<void(unsigned)> fn) throw()
+{
+	threads::alock h(lock);
+	fn(last_complete);
+}
+
 }