Cleanup: remove unused includes in Compositor

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

Slightly reduces compile time.

source/blender/compositor/operations/COM_VariableSizeBokehBlurOperation.cc

@@ -17,12 +17,8 @@
  */
 
 #include "COM_VariableSizeBokehBlurOperation.h"
-#include "BLI_math.h"
-#include "COM_ExecutionSystem.h"
 #include "COM_OpenCLDevice.h"
 
-#include "RE_pipeline.h"
-
 namespace blender::compositor {
 
 VariableSizeBokehBlurOperation::VariableSizeBokehBlurOperation()