Remove type inference casts from FFI code

[tiago-lqt]

Remove all pointer casts using type inference from code interacting with the openexr-core-sys crate.

The current Drop function for Reader has an type cast using type inference.
This caused casting the *const sys::exr_file_t returned by self.inner() into *mut *mut exr_file_t required by exr_close(). Which lead to a crash at runtime when dropping a Reader struct.