emile: unmap the memory for image file after decode.

emile: unmap the memory for image file after decode.

Summary:
In case of emile, unmap the memory of the image file when the image file is closing.
But closing of image file happen when the image is flushed from the cache, so the
mapped memory of image file remain for long period of time with no benefit (If it is
in cache, it won't get decoded again). So I think we don't need to keep the mapped
memory of image file.

Test Plan: self

Reviewers: cedric, jpeg, jypark

Reviewed By: cedric

Differential Revision: https://phab.enlightenment.org/D5210

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