evas: start work on making the loader module a public API.

evas: start work on making the loader module a public API.

Goal is to be able to remove all internal Evas call from inside all
loader module. To do so we are going to open and hold a reference to the
file from outside of the module, read the header, create the image data,
load the data, close that reference.

Once that done, the next step is to let the file remain open as soon as
the filename/key is set and add an API to set an Eina_File directly. This
way edje can maintain the same file open as it use for an edje object,
keeping things in sync and avoid rendering glitch during update.