Refactor: Make display_clear a base render method

Should be no functional changes.

source/blender/render/intern/pipeline.cc

@@ -1779,7 +1779,7 @@ static bool render_init_from_main(Render *re,
   ntreeCompositTagRender(re->scene);
 
   re->display_init_cb(re->dih, re->result);
-  re->display_clear_cb(re->dch, re->result);
+  re->display_clear(re->result);
 
   return true;
 }

source/blender/render/intern/render_types.cc

@@ -41,6 +41,13 @@ Render::~Render()
   render_result_free(pushedresult);
 }
 
+void Render::display_clear(RenderResult *render_result)
+{
+  if (display_clear_cb) {
+    display_clear_cb(dch, render_result);
+  }
+}
+
 void Render::display_update(RenderResult *render_result, rcti *rect)
 {
   if (display_update_cb) {

source/blender/render/intern/render_types.h

@@ -49,6 +49,7 @@ struct BaseRender {
                                   const char *view_name) = 0;
   virtual void compositor_free() = 0;
 
+  virtual void display_clear(RenderResult *render_result) = 0;
   virtual void display_update(RenderResult *render_result, rcti *rect) = 0;
   virtual void current_scene_update(struct Scene *scene) = 0;
 
@@ -100,6 +101,7 @@ struct ViewRender : public BaseRender {
   }
   void compositor_free() override {}
 
+  void display_clear(RenderResult * /*render_result*/) override {}
   void display_update(RenderResult * /*render_result*/, rcti * /*rect*/) override {}
   void current_scene_update(struct Scene * /*scene*/) override {}
 
@@ -140,6 +142,7 @@ struct Render : public BaseRender {
                           const char *view_name) override;
   void compositor_free() override;
 
+  void display_clear(RenderResult *render_result) override;
   void display_update(RenderResult *render_result, rcti *rect) override;
   void current_scene_update(struct Scene *scene) override;