Cleanup c2rust output in `mc.rs`

The fallback functions in mc.rs have already been partially cleaned up, but they still make heavy use of raw pointer operations, even in places where slices could be easily used.

[x] Cleanup helper functions to take safe arguments where possible (may take raw pointers as arguments currently).
[x] General cleanup of c2rust generated code in functions.
[x] Restore comments from original C.

The following functions still need to be cleaned:

[x] put_rust
[x] prep_rust
[x] filter_8tap
[x] put_8tap_rust
[x] put_8tap_scaled_rust
[x] prep_8tap_rust
[x] prep_8tap_scaled_rust
[x] filter_bilin
[x] put_bilin_rust
[x] put_bilin_scaled_rust
[x] prep_bilin_rust
[x] prep_bilin_scaled_rust
[x] avg_rust
[x] w_avg_rust
[x] mask_rust
[x] blend_rust
[x] blend_v_rust
[x] blen_h_rust
[x] w_mask_rust
[x] warp_affine_8x8_rust
[x] warp_affine_8x8t_rust
[x] emu_edge_rust
[x] resize_rust

memorysafety / rav1d

Cleanup c2rust output in `mc.rs` #819