evas: opaque value does not change even if image data be changed

evas: opaque value does not change even if image data be changed

Summary:
opaque value keeps previous image's value
when image data be changed by file_set/mmap_set api

@fix

Test Plan:
1) Make a evas rectangle object has color
2) Make a evas image object with jpg file.
3) Replace image data with png file by evas_object_image_file_set.

Reviewers: Hermet, cedric, jypark

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D2774

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>