A couple of render improvements;

- Bug fix: the upper tile in a collumn for Panorama render didn't put the
  mainthread to sleep properly. Now panorama renders 25% faster if you had
  set Y-Parts to 4.

- Enabling Compositing in Scene for first time now adds a "Composite" node
  too, so render output gets applied.
- An attempt to render with "Do Composite" without "Composite" node will
  throw an error and stops rendering. In background mode it will just not
  render at all, and print errors.
- Errors that prevent rendering now give a popup menu again.
- Having MBlur or Fields option on will now normally render, but with an
  error print in console (not done yet...)

source/blender/src/toolbox.c

@@ -221,7 +221,7 @@ void error(char *fmt, ...)
 	sprintf(nfmt, "%s", fmt);
 	
 	va_start(ap, fmt);
-	if (G.background || !G.curscreen || (G.rendering)) {
+	if (G.background || !G.curscreen) {
 		vprintf(nfmt, ap);
 		printf("\n");
 	} else {