Fix filter::filter::PadIter to actually iterate

Bug caught by clippy:

  error: offset calculation on zero-sized value
    --> src/filter/filter.rs:98:33
     |
  98 |             let pad = Pad::wrap(self.ptr.offset(self.cur));
     |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `#[deny(clippy::zst_offset)]` on by default
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#zst_offset

The problem is that `AVFilterPad` is zero-sized:

  #[repr(C)]
  #[derive(Debug, Copy, Clone)]
  pub struct AVFilterPad {
      _unused: [u8; 0],
  }

which is in turn due to `AVFilterPad` being an opaque type in
`libavfilter/avfilter.h`:

  typedef struct AVFilterContext AVFilterContext;
  typedef struct AVFilterLink    AVFilterLink;
  typedef struct AVFilterPad     AVFilterPad;
  typedef struct AVFilterFormats AVFilterFormats;

Doing pointer arithmetic on an opaque (incomplete) type doesn't work. We have
to use the proper FFI calls to obtain info on individual pads. The API has to
be tweaked a bit; hopefully it doesn't break user programs (if it does it
should only break bugged ones...).

Fixes #20.

src/filter/mod.rs

@@ -58,3 +58,21 @@ pub fn find(name: &str) -> Option<Filter> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_paditer() {
+        assert_eq!(
+            find("overlay")
+                .unwrap()
+                .inputs()
+                .unwrap()
+                .map(|input| input.name().unwrap().to_string())
+                .collect::<Vec<_>>(),
+            vec!("main", "overlay")
+        );
+    }
+}