Fix accelerator structure descriptor methods

These return a +0 autoreleased pointer, so we have to retain them
ourselves before we put them in the strong smart pointer created by
`foreign_obj_type!`.

src/accelerator_structure.rs

@@ -81,7 +81,9 @@ impl PrimitiveAccelerationStructureDescriptor {
  pub fn descriptor() -> Self {
  unsafe {
  let class = class!(MTLPrimitiveAccelerationStructureDescriptor);
- msg_send![class, descriptor]
+ let ptr: *mut Object = msg_send![class, descriptor];
+ let ptr: *mut Object = msg_send![ptr, retain];
+ Self::from_ptr(ptr as _)
  }
  }
 }
@@ -150,7 +152,9 @@ impl AccelerationStructureTriangleGeometryDescriptor {
  pub fn descriptor() -> Self {
  unsafe {
  let class = class!(MTLAccelerationStructureTriangleGeometryDescriptor);
- msg_send![class, descriptor]
+ let ptr: *mut Object = msg_send![class, descriptor];
+ let ptr: *mut Object = msg_send![ptr, retain];
+ Self::from_ptr(ptr as _)
  }
  }
 }
@@ -209,7 +213,9 @@ impl AccelerationStructureBoundingBoxGeometryDescriptor {
  pub fn descriptor() -> Self {
  unsafe {
  let class = class!(MTLAccelerationStructureBoundingBoxGeometryDescriptor);
- msg_send![class, descriptor]
+ let ptr: *mut Object = msg_send![class, descriptor];
+ let ptr: *mut Object = msg_send![ptr, retain];
+ Self::from_ptr(ptr as _)
  }
  }
 }
@@ -236,7 +242,9 @@ impl InstanceAccelerationStructureDescriptor {
  pub fn descriptor() -> Self {
  unsafe {
  let class = class!(MTLInstanceAccelerationStructureDescriptor);
- msg_send![class, descriptor]
+ let ptr: *mut Object = msg_send![class, descriptor];
+ let ptr: *mut Object = msg_send![ptr, retain];
+ Self::from_ptr(ptr as _)
  }
  }
 }
@@ -282,7 +290,9 @@ impl IndirectInstanceAccelerationStructureDescriptor {
  pub fn descriptor() -> Self {
  unsafe {
  let class = class!(MTLIndirectInstanceAccelerationStructureDescriptor);
- msg_send![class, descriptor]
+ let ptr: *mut Object = msg_send![class, descriptor];
+ let ptr: *mut Object = msg_send![ptr, retain];
+ Self::from_ptr(ptr as _)
  }
  }
 }