Process pending tasks before shutting down RenderEngineThreaded

This ensures texture destruction doesn't race during shutdown, which
surfaced when adding RE's Vulkan backends to librenderengine_bench

Bug: b/425756879
Test: Vulkan backends crash librenderengine_test without this change
Flag: EXEMPT RE instances are only destroyed in tests and benchmarks
Change-Id: Iacaa875c598a54f5e4d5245144af0fcf385e202f