Cleanup: remove unused includes in Compositor

And move unneeded includes in frequently used headers
to source files.

Slightly reduces compile time.

source/blender/compositor/nodes/COM_ZCombineNode.cc

@@ -18,15 +18,9 @@
 
 #include "COM_ZCombineNode.h"
 
-#include "COM_ZCombineOperation.h"
-
 #include "COM_AntiAliasOperation.h"
-#include "COM_ExecutionSystem.h"
 #include "COM_MathBaseOperation.h"
-#include "COM_MixOperation.h"
-#include "COM_SetValueOperation.h"
-
-#include "DNA_material_types.h" /* the ramp types */
+#include "COM_ZCombineOperation.h"
 
 namespace blender::compositor {