software_generic: Add plane assign/release function pointers

Functions to assign a plane for a native surface, and release a plane
that's been assigned to a native surface.

These are empty for now as they'll need to be overridden in any backend
that can handle planes.

src/lib/evas/include/evas_private.h

@@ -1482,6 +1482,9 @@ struct _Evas_Func
    void (*image_cache_set)                 (void *engine, int bytes);
    int  (*image_cache_get)                 (void *engine);
 
+   void *(*image_plane_assign)             (void *data, void *image, int x, int y);
+   void (*image_plane_release)             (void *data, void *image, void *plane);
+
    Evas_Font_Set *(*font_load)             (void *engine, const char *name, int size, Font_Rend_Flags wanted_rend);
    Evas_Font_Set *(*font_memory_load)      (void *engine, const char *source, const char *name, int size, const void *fdata, int fdata_size, Font_Rend_Flags wanted_rend);
    Evas_Font_Set *(*font_add)              (void *engine, Evas_Font_Set *font, const char *name, int size, Font_Rend_Flags wanted_rend);

src/modules/evas/engines/software_generic/evas_engine.c

@@ -4745,6 +4745,8 @@ static Evas_Func func =
      eng_image_cache_flush,
      eng_image_cache_set,
      eng_image_cache_get,
+     NULL, // image_plane_assign
+     NULL, // image_plane_release
      /* font draw functions */
      eng_font_load,
      eng_font_memory_load,