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/renderwin.c

@@ -80,6 +80,7 @@
 #include "BIF_renderwin.h"
 #include "BIF_resources.h"
 #include "BIF_toets.h"
+#include "BIF_toolbox.h"
 #include "BIF_writeimage.h"
 
 #include "BDR_editobject.h"
@@ -1030,6 +1031,7 @@ static void end_test_break_callback()
    - set callbacks
    - cleanup
 */
+static void error_cb(char *str){error(str);}
 
 static void do_render(int anim)
 {
@@ -1045,6 +1047,7 @@ static void do_render(int anim)
 	RE_display_init_cb(re, renderwin_init_display_cb);
 	RE_display_draw_cb(re, renderwin_progress_display_cb);
 	RE_display_clear_cb(re, renderwin_clear_display_cb);
+	RE_error_cb(re, error_cb);
 	init_test_break_callback();
 	RE_test_break_cb(re, test_break);
 	RE_timecursor_cb(re, set_timecursor);