xrt: Introduce xrt_comp_fd_destroy to avoid some error prone casting

src/xrt/compositor/client/comp_egl_glue.c

@@ -31,7 +31,7 @@ client_egl_compositor_destroy(struct xrt_compositor *xc)
 {
 	struct client_gl_compositor *c = client_gl_compositor(xc);
 	// Pipe down call into fd compositor.
-	xrt_comp_destroy((struct xrt_compositor **)c->xcfd);
+	xrt_comp_fd_destroy(&c->xcfd);
 	free(c);
 }
 

src/xrt/compositor/client/comp_gl_xlib_client.c

@@ -33,7 +33,7 @@ client_gl_xlib_compositor_destroy(struct xrt_compositor *xc)
 {
 	struct client_gl_xlib_compositor *c = client_gl_xlib_compositor(xc);
 	// Pipe down call into fd compositor.
-	xrt_comp_destroy((struct xrt_compositor **)c->base.xcfd);
+	xrt_comp_fd_destroy(&c->base.xcfd);
 	free(c);
 }
 

src/xrt/compositor/client/comp_vk_client.c

@@ -116,7 +116,7 @@ client_vk_compositor_destroy(struct xrt_compositor *xc)
 	}
 
 	// Pipe down call into fd compositor.
-	xrt_comp_destroy((struct xrt_compositor **)&c->xcfd);
+	xrt_comp_fd_destroy(&c->xcfd);
 	free(c);
 }
 

src/xrt/include/xrt/xrt_compositor.h

@@ -777,6 +777,27 @@ xrt_comp_fd_create_swapchain(struct xrt_compositor_fd *xcfd,
 	return (struct xrt_swapchain_fd *)xsc;
 }
 
+/*!
+ * @copydoc xrt_compositor::destroy
+ *
+ * Helper for calling through the function pointer: does a null check and sets
+ * xcfd_ptr to null if freed.
+ *
+ * @public @memberof xrt_compositor_fd
+ */
+static inline void
+xrt_comp_fd_destroy(struct xrt_compositor_fd **xcfd_ptr)
+{
+	struct xrt_compositor_fd *xcfd = *xcfd_ptr;
+	if (xcfd == NULL) {
+		return;
+	}
+
+	xcfd->base.destroy(&xcfd->base);
+	*xcfd_ptr = NULL;
+}
+
+
 #ifdef __cplusplus
 }
 #endif