Compositor: Avoid redundant executions for only previews

The compositor executes redundantly if previews are enabled but no
other output is computed. This shouldn't be the case because previews
are secondary outputs that need a primary output to compute with.

We improve this this by only executing the compositor if a primary
output is needed, regardless if previews are needed or not.

Pull Request: https://projects.blender.org/blender/blender/pulls/140920

source/blender/editors/space_node/node_edit.cc

@@ -38,6 +38,7 @@
 #include "BLI_math_vector.hh"
 #include "BLI_string.h"
 #include "BLI_string_utf8.h"
+#include "BLI_utildefines.h"
 
 #include "BLT_translation.hh"
 
@@ -436,8 +437,14 @@ static blender::compositor::OutputTypes get_compositor_needed_outputs(const bCon
 
 void ED_node_composite_job(const bContext *C, bNodeTree *nodetree, Scene *scene_owner)
 {
+  /* None of the outputs are needed except maybe previews, so no need to execute the compositor.
+   * Previews are not considered because they are a secondary output that needs another output to
+   * be computed with. */
   blender::compositor::OutputTypes needed_outputs = get_compositor_needed_outputs(C);
-  if (needed_outputs == blender::compositor::OutputTypes::None) {
+  if (ELEM(needed_outputs,
+           blender::compositor::OutputTypes::None,
+           blender::compositor::OutputTypes::Previews))
+  {
     return;
   }