loaders: GP_Loader: Get rid of unneeded members

This commit cleanups a few aspects of loaders implementation.

It creates GP_LoaderLoadImage() function that can be used to load
image from path for a given loader pointer which allows us to.

* Get rid of the Load() callback from GP_Loader

* Unify GP_LoadFOO implementation

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
build: Fix Debian build, core should link with -lrt

Signed-off-by: Cyril Hrubis <metan@ucw.cz>