Control extra space for images conversion in image_cache

Size of image converted to raw format can be growing up to virtual_size.
This patch adds new _fetch_to_raw function to image_cache which try to
free extra space for image conversion if needed.

Closes-Bug: #1356362
Change-Id: I69712b078113ef0640859389ef4ab9039135f612
Merge "Control extra space for images conversion in image_cache"