fn dav1d_submit_frame: Cleanup mostly (#431)

There's still some more things left to clean up, but they're more
cross-cutting so I'm saving them until later.

include/dav1d/common.rs

@@ -1,3 +1,5 @@
+use std::ptr;
+
 use crate::include::stddef::size_t;
 use crate::include::stdint::int64_t;
 use crate::include::stdint::uint8_t;
@@ -10,7 +12,16 @@ pub struct Dav1dUserData {
     pub r#ref: *mut Dav1dRef,
 }
 
-#[derive(Clone)]
+impl Default for Dav1dUserData {
+    fn default() -> Self {
+        Self {
+            data: ptr::null(),
+            r#ref: ptr::null_mut(),
+        }
+    }
+}
+
+#[derive(Clone, Default)]
 #[repr(C)]
 pub struct Dav1dDataProps {
     pub timestamp: int64_t,

include/dav1d/data.rs

@@ -1,3 +1,5 @@
+use std::ptr;
+
 use crate::include::dav1d::common::Dav1dDataProps;
 use crate::include::stddef::size_t;
 use crate::include::stdint::uint8_t;
@@ -11,3 +13,14 @@ pub struct Dav1dData {
     pub r#ref: *mut Dav1dRef,
     pub m: Dav1dDataProps,
 }
+
+impl Default for Dav1dData {
+    fn default() -> Self {
+        Self {
+            data: ptr::null(),
+            sz: Default::default(),
+            r#ref: ptr::null_mut(),
+            m: Default::default(),
+        }
+    }
+}