gl: wrapper for the gst_gl_context_thread_add function

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1313>

gstreamer-gl/Gir.toml

@@ -300,8 +300,7 @@ manual_traits = ["GLContextExtManual"]
 
     [[object.function]]
     name = "thread_add"
-    # unimplemented GLContextThreadFunc
-    ignore = true
+    manual = true
 
     [[object.function]]
     name = "get_thread"

gstreamer-gl/src/gl_context.rs

@@ -67,6 +67,29 @@ pub trait GLContextExtManual: sealed::Sealed + IsA<GLContext> + 'static {
             ) as uintptr_t
         }
     }
+
+    #[doc(alias = "gst_gl_context_thread_add")]
+    fn thread_add<F: FnOnce(&Self) + Send>(&self, func: F) {
+        let mut func = std::mem::ManuallyDrop::new(func);
+        let user_data: *mut F = &mut *func;
+
+        unsafe extern "C" fn trampoline<O: IsA<GLContext>, F: FnOnce(&O) + Send>(
+            context: *mut ffi::GstGLContext,
+            data: glib::ffi::gpointer,
+        ) {
+            let func = std::ptr::read(data as *mut F);
+            let context = GLContext::from_glib_borrow(context);
+            func(context.unsafe_cast_ref())
+        }
+
+        unsafe {
+            ffi::gst_gl_context_thread_add(
+                self.as_ref().to_glib_none().0,
+                Some(trampoline::<Self, F>),
+                user_data as glib::ffi::gpointer,
+            );
+        }
+    }
 }
 
 impl<O: IsA<GLContext>> GLContextExtManual for O {}