Move GPU_Flip calls into CVideo::flip().

src/loadscreen.cpp

@@ -234,7 +234,7 @@ void loadscreen::draw_screen(const std::string &text)
 		GPU_FreeImage(img);
 	}
 
-	GPU_Flip(get_render_target());
+	screen_.flip();
 #else
 	surface gdis = screen_.getSurface();
 	SDL_Rect area;

src/storyscreen/render.cpp

@@ -357,7 +357,7 @@ bool part_ui::render_floating_images()
 
 		if(!ri.image.null()) {
 			ri.image.draw(*target, ri.rect.x, ri.rect.y);
-			GPU_Flip(target);
+			video_.flip();
 		}
 
 		if (!skip_)
@@ -644,7 +644,7 @@ void part_ui::render_story_box()
 
 	const std::string& storytxt = p_.text();
 	if(storytxt.empty()) {
-		GPU_Flip(target);
+		video_.flip();
 		wait_for_input();
 		return;
 	}
@@ -735,7 +735,7 @@ void part_ui::render_story_box()
 			dstrect.y = fix_text_y + scan.y + storybox_padding;
 			txttxt.set_clip(scan);
 			txttxt.draw(*target, dstrect.x, dstrect.y);
-			GPU_Flip(target);
+			video_.flip();
 			++scan.y;
 		}
 		else skip_ = true;

src/video.cpp

@@ -510,7 +510,10 @@ void CVideo::flip()
 {
 	if(fake_screen_)
 		return;
-
+#ifdef SDL_GPU
+	assert(render_target);
+	GPU_Flip(render_target);
+#else
 #if !SDL_VERSION_ATLEAST(2, 0, 0)
 	if(update_all) {
 		::SDL_Flip(frameBuffer);
@@ -526,6 +529,7 @@ void CVideo::flip()
 	assert(window);
 	window->render();
 #endif
+#endif
 }
 
 void CVideo::lock_updates(bool value)