move debug flag into its own global var (G.debug), split up debug options.

--debug
  --debug-ffmpeg
  --debug-python
  --debug-events
  --debug-wm

This makes debug output easier to read - event debug prints would flood output too much before.

For convenience:
  --debug-all turns all debug flags on (works as --debug did before).

also removed some redundant whitespace in debug prints and prefix some prints with __func__ to give some context.

source/blender/nodes/composite/node_composite_tree.c

@@ -523,7 +523,7 @@ static void freeExecutableNode(bNodeTreeExec *exec)
 				if (ns && ns->data) {
 					free_compbuf(ns->data);
 					ns->data= NULL;
-					// printf("freed buf node %s \n", node->name);
+					// printf("freed buf node %s\n", node->name);
 				}
 			}
 		}