video/video_rectangle: Mark struct as repr(C) for FFI transparency

This struct is "reimplemented" to make it easier to access the fields
instead of going through `ffi::GstVideoRectangle`, but should be marked
`repr(C)` to remain binary-compatible.

gstreamer-video/src/video_rectangle.rs

@@ -3,6 +3,7 @@
 use glib::translate::IntoGlib;
 use std::mem;
 
+#[repr(C)]
 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
 pub struct VideoRectangle {
     pub x: i32,