Cleanup: Stop accessing gpu_batch_presets_reset()

The current code is accessing this from outside the gpu "namespace". As
such it should be accessing GPU_ functions, not gpu_ functions.

This is also a place to centralize the XXX message that will be
addressed upon refactor. So we can reuse this call in other places that
need the same temporary workaround.

Groundwork for upcoming fix (D8472)

source/blender/editors/space_image/space_image.c

@@ -643,9 +643,7 @@ static void image_main_region_draw(const bContext *C, ARegion *region)
   // View2DScrollers *scrollers;
   float col[3];
 
-  /* XXX This is in order to draw UI batches with the DRW
-   * old context since we now use it for drawing the entire area. */
-  gpu_batch_presets_reset();
+  GPU_batch_presets_reset();
 
   GPUViewport *viewport = region->draw_buffer->viewport;
   DefaultFramebufferList *fbl = GPU_viewport_framebuffer_list_get(viewport);

source/blender/gpu/GPU_batch_presets.h

@@ -50,6 +50,8 @@ bool gpu_batch_presets_unregister(struct GPUBatch *preset_batch);
 void gpu_batch_presets_reset(void);
 void gpu_batch_presets_exit(void);
 
+void GPU_batch_presets_reset(void);
+
 #ifdef __cplusplus
 }
 #endif

source/blender/gpu/intern/gpu_batch_presets.c

@@ -406,4 +406,17 @@ void gpu_batch_presets_exit(void)
   BLI_mutex_end(&g_presets_3d.mutex);
 }
 
+/**
+ * This function only needs to be accessed externally because
+ * we are drawing UI batches with the DRW old context.
+ *
+ * And now we use it for drawing the entire area.
+ *
+ * XXX (Clément) - to cleanup in the upcoming 2.91 refactor.
+ **/
+void GPU_batch_presets_reset()
+{
+  gpu_batch_presets_reset();
+}
+
 /** \} */