Casting to more strictly aligned pointer types

[hatzel]

These clippy lints need looking into:

error: casting from `*mut ffmpeg::AVDictionary` to a more-strictly-aligned pointer (`*mut worker::util::Dictionary`)
  --> src/worker/mediafile.rs:77:29
   |
77 |         let d = dict_to_map(av.metadata as *mut Dictionary);
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[deny(cast_ptr_alignment)] on by default
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#cast_ptr_alignment

error: casting from `*mut ffmpeg::AVDictionary` to a more-strictly-aligned pointer (`*mut worker::util::Dictionary`)
   --> src/worker/mediafile.rs:252:34
    |
252 |             let md = dict_to_map((*self.ctx).metadata as *mut Dictionary);
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#cast_ptr_alignment

error: aborting due to 2 previous errors