Cleanup: replace members m_ prefix by _ suffix in Compositor

To convert old code to the current convention and
use a single code style.

source/blender/compositor/operations/COM_SetValueOperation.h

@@ -28,7 +28,7 @@ namespace blender::compositor {
  */
 class SetValueOperation : public ConstantOperation {
  private:
-  float m_value;
+  float value_;
 
  public:
   /**
@@ -38,16 +38,16 @@ class SetValueOperation : public ConstantOperation {
 
   const float *get_constant_elem() override
   {
-    return &m_value;
+    return &value_;
   }
 
   float getValue()
   {
-    return m_value;
+    return value_;
   }
   void setValue(float value)
   {
-    m_value = value;
+    value_ = value;
   }
 
   /**