Made raw/owned fields private

* Removed `pub` modifier from these fields
* Implemented accessors for these fields where appropriate
* Implemented `new_from_raw` unsafe constructors because they are
  sometimes needed
Made impl_raw_constructors more general
Changed `new_from_raw` to `from_ll` for consistency
Merge branch 'master' of https://github.com/dpx-infinity/rust-sdl2 into dpx-infinity-master

Conflicts:
	src/sdl2/render.rs