message: Repair backend unit tests on macOS.
  This fixes "illegal hardware instruction" seen on my MacBook, which
  is the same as but triggered only in a test environment where
  the main thread is the same as the driver thread, and filament is built
  with HAS_THREADING enabled.
diff: @@ -151,7 +151,7 @@ void PlatformCocoaGLImpl::updateOpenGLContext(NSView *nsView, bool resetView) {
  // "update" to be called from the UI thread. This became a hard requirement with the arrival
  // of macOS 10.15 (Catalina). If we were to call these methods from the GL thread, we would
  // see EXC_BAD_INSTRUCTION.
  - #if UTILS_HAS_THREADING
  + if (![NSThread isMainThread]) {
  dispatch_sync(dispatch_get_main_queue(), ^(void) {
  if (resetView) {
  [glContext clearDrawable];
  @@ -159,13 +159,13 @@ void PlatformCocoaGLImpl::updateOpenGLContext(NSView *nsView, bool resetView) {
  }
  [glContext update];
  });
  - #else
  + } else {
  if (resetView) {
  [glContext clearDrawable];
  [glContext setView:nsView];
  }
  [glContext update];
  - #endif
  + }
  }
  } // namespace filament
  