efl_ui_bg: disable preloading of the internal image

efl_ui_bg: disable preloading of the internal image

Summary:
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
elm_bg.

Test Plan: elm_bg_file_set with big image.

Reviewers: Hermet

Reviewed By: Hermet

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D11898