Fix skipFrame incorrectly asserts in some cases

BUGS=[357992376]

filament/src/details/Renderer.cpp

@@ -344,10 +344,14 @@ bool FRenderer::beginFrame(FSwapChain* swapChain, uint64_t vsyncSteadyClockTimeN
     * to ignore the return value and render the frame anyway -- which is perfectly fine.
     * The remaining work will be done when the first render() call is made.
     */
-    auto beginFrameInternal = [this, appVsync]() {
+    auto beginFrameInternal = [this, appVsync, swapChain]() {
         FEngine& engine = mEngine;
         FEngine::DriverApi& driver = engine.getDriverApi();
 
+        // we need to re-set mSwapChain here because if a frame was marked as "skip" but the
+        // user ignored us, we'd still want mSwapChain to be set.
+        mSwapChain = swapChain;
+
         driver.beginFrame(
                 appVsync.time_since_epoch().count(),
                 mDisplayInfo.refreshRate == 0.0 ? 0 : int64_t(
@@ -376,6 +380,10 @@ bool FRenderer::beginFrame(FSwapChain* swapChain, uint64_t vsyncSteadyClockTimeN
     // so we need to delay this work until that happens.
     mBeginFrameInternal = beginFrameInternal;
 
+    // clear mSwapChain because the frame should be skipped (it will be re-set if the user
+    // ignores the skip)
+    mSwapChain = nullptr;
+
     // we need to flush in this case, to make sure the tick() call is executed at some point
     engine.flush();
 
@@ -534,8 +542,6 @@ void FRenderer::renderStandaloneView(FView const* view) {
 void FRenderer::render(FView const* view) {
     SYSTRACE_CALL();
 
-    assert_invariant(mSwapChain);
-
     if (UTILS_UNLIKELY(mBeginFrameInternal)) {
         // this should not happen, the user should not call render() if we returned false from
         // beginFrame(). But because this is allowed, we handle it gracefully.
@@ -543,6 +549,9 @@ void FRenderer::render(FView const* view) {
         mBeginFrameInternal = {};
     }
 
+    // after beginFrame() is called, mSwapChain should be true
+    assert_invariant(mSwapChain);
+
     if (UTILS_LIKELY(view && view->getScene() && view->hasCamera())) {
         if (mViewRenderedCount) {
             // This is a good place to kick the GPU, since we've rendered a View before,