don't require SwapChain to be passed to viewer.render - this will just render into all registered swapchains

thermion_dart/lib/src/viewer/src/ffi/src/thermion_viewer_ffi.dart

@@ -123,25 +123,29 @@ class ThermionViewerFFI extends ThermionViewer {
   ///
   ///
   @override
-  Future render(SwapChain swapchain) async {
-    await withBoolCallback(
-      (cb) => Renderer_beginFrameRenderThread(
-        app.renderer,
-        (swapchain as FFISwapChain).swapChain,
-        0.toBigInt,
-        cb,
-      ),
-    );
+  Future render() async {
+    final swapChains =
+        await (FilamentApp.instance as FFIFilamentApp).getSwapChains();
+    for (final swapChain in swapChains) {
+      await withBoolCallback(
+        (cb) => Renderer_beginFrameRenderThread(
+          app.renderer,
+          swapChain.getNativeHandle(),
+          0.toBigInt,
+          cb,
+        ),
+      );
 
-    await withVoidCallback(
-      (requestId, cb) =>
-          Renderer_renderRenderThread(app.renderer, view.view, requestId, cb),
-    );
-    await withVoidCallback(
-      (requestId, cb) =>
-          Renderer_endFrameRenderThread(app.renderer, requestId, cb),
-    );
-    await FilamentApp.instance!.flush();
+      await withVoidCallback(
+        (requestId, cb) =>
+            Renderer_renderRenderThread(app.renderer, view.view, requestId, cb),
+      );
+      await withVoidCallback(
+        (requestId, cb) =>
+            Renderer_endFrameRenderThread(app.renderer, requestId, cb),
+      );
+      await FilamentApp.instance!.flush();
+    }
   }
 
   double _msPerFrame = 1000.0 / 60.0;

thermion_dart/lib/src/viewer/src/thermion_viewer_base.dart

@@ -39,9 +39,9 @@ abstract class ThermionViewer {
   Future setRendering(bool render);
 
   ///
-  /// Render a single frame immediately.
+  /// Render a single frame immediately into all registered swapchains.
   ///
-  Future render(SwapChain swapChain);
+  Future render();
 
   ///
   ///