efl_ui_bg: disable preloading of the internal image
preloading is elm_image feature. it is better to disable preloading for elm_bg
becuase there is no way to disable preloading of internal image in elm_bg.
If API user want to use preloaded image, elm_image is recommended instead of
Test Plan: elm_bg_file_set with big image.
Reviewed By: Hermet
Differential Revision: https://phab.enlightenment.org/D11898