struct Rav1dTaskContext: Remove *const Rav1dContext field (#721)

`Rav1dTaskContext` doesn't need to hold a pointer to its parent if we
pass this borrow into the thread start function explicitly.

src/internal.rs

@@ -665,7 +665,6 @@ pub(crate) struct Rav1dTaskContext_task_thread {
 
 #[repr(C)]
 pub(crate) struct Rav1dTaskContext {
-    pub c: *const Rav1dContext,
     pub f: *const Rav1dFrameContext,
     pub ts: *mut Rav1dTileState,
     pub bx: c_int,
@@ -692,3 +691,12 @@ pub(crate) struct Rav1dTaskContext {
     pub frame_thread: Rav1dTaskContext_frame_thread,
     pub task_thread: Rav1dTaskContext_task_thread,
 }
+
+// TODO(SJC): This is a temporary struct to pass a single pointer that holds
+// both a Rav1dContext and Rav1dTaskContext to the start routine in
+// pthread_create. We need to pass the Rav1dTaskContext into the thread by value
+// and remove it from the context structure.
+pub(crate) struct Rav1dTaskContext_borrow<'c> {
+    pub c: &'c Rav1dContext,
+    pub tc: &'c mut Rav1dTaskContext,
+}

src/lib.rs

@@ -38,6 +38,7 @@ use crate::src::internal::Rav1dContext;
 use crate::src::internal::Rav1dFrameContext;
 use crate::src::internal::Rav1dTask;
 use crate::src::internal::Rav1dTaskContext;
+use crate::src::internal::Rav1dTaskContext_borrow;
 use crate::src::internal::TaskThreadData;
 use crate::src::intra_edge::rav1d_init_mode_tree;
 use crate::src::levels::Av1Block;
@@ -375,15 +376,18 @@ pub(crate) unsafe fn rav1d_open(c_out: &mut *mut Rav1dContext, s: &Rav1dSettings
         (*t).f = &mut *((*c).fc).offset(0) as *mut Rav1dFrameContext;
         (&mut (*t).task_thread.ttd as *mut Arc<TaskThreadData>)
             .write(Arc::clone(&(*c).task_thread));
-        (*t).c = c;
         *BitDepth16::select_mut(&mut (*t).cf) = Align64([0; 32 * 32]);
         if (*c).n_tc > 1 as c_uint {
             (*t).task_thread.td.cond = Condvar::new();
+            let thread_args = Box::new(Rav1dTaskContext_borrow {
+                c: &*c,
+                tc: &mut *t,
+            });
             if pthread_create(
                 &mut (*t).task_thread.td.thread,
                 &mut thread_attr,
                 Some(rav1d_worker_task),
-                t as *mut c_void,
+                Box::into_raw(thread_args).cast(),
             ) != 0
             {
                 return error(c, c_out, &mut thread_attr);

src/thread_task.rs

@@ -20,6 +20,7 @@ use crate::src::internal::Rav1dContext;
 use crate::src::internal::Rav1dFrameContext;
 use crate::src::internal::Rav1dTask;
 use crate::src::internal::Rav1dTaskContext;
+use crate::src::internal::Rav1dTaskContext_borrow;
 use crate::src::internal::Rav1dTileState;
 use crate::src::internal::TaskThreadData;
 use crate::src::internal::TaskThreadData_delayed_fg;
@@ -790,8 +791,7 @@ unsafe fn delayed_fg_task<'l, 'ttd: 'l>(
 }
 
 pub unsafe extern "C" fn rav1d_worker_task(data: *mut c_void) -> *mut c_void {
-    let tc: &mut Rav1dTaskContext = &mut *(data as *mut Rav1dTaskContext);
-    let c: &Rav1dContext = &*tc.c;
+    let Rav1dTaskContext_borrow { c, tc } = *Box::from_raw(data as *mut Rav1dTaskContext_borrow);
 
     // We clone the Arc here for the lifetime of this function to avoid an
     // immutable borrow of tc across the call to park