add next pointer chain infrastructure

Signed-off-by: Schmarni <marnistromer@gmail.com>

crates/bevy_openxr/src/openxr/next_chain.rs

@@ -0,0 +1,44 @@
+use openxr::sys;
+use std::{ffi::c_void, ptr};
+
+/// An abstraction for the next pointer fields for openxr calls
+#[derive(Default)]
+pub struct OxrNextChain {
+    structs: Vec<Box<dyn OxrNextChainStructProvider>>,
+}
+
+impl OxrNextChain {
+    pub fn push<T: OxrNextChainStructProvider>(&mut self, info_struct: T) {
+        if let Some(last) = self.structs.last_mut() {
+            let mut info = Box::new(info_struct);
+            info.as_mut().clear_next();
+            last.as_mut().set_next(info.as_ref().header());
+            self.structs.push(info);
+        } else {
+            let mut info_struct = Box::new(info_struct);
+            info_struct.as_mut().clear_next();
+            self.structs.push(info_struct);
+        }
+    }
+    pub fn chain(&self) -> Option<&OxrNextChainStructBase> {
+        self.structs.first().map(|v| v.as_ref().header())
+    }
+    pub fn chain_pointer(&self) -> *const c_void {
+        self.chain()
+            .map(|v| v as *const _ as *const c_void)
+            .unwrap_or(ptr::null())
+    }
+}
+
+pub trait OxrNextChainStructProvider: 'static {
+    fn header(&self) -> &OxrNextChainStructBase;
+    fn set_next(&mut self, next: &OxrNextChainStructBase);
+    fn clear_next(&mut self);
+}
+
+#[repr(C)]
+#[derive(Copy, Clone, Debug)]
+pub struct OxrNextChainStructBase {
+    pub ty: sys::StructureType,
+    pub next: *const OxrNextChainStructBase,
+}