More speedup of scrolling when many windowed plugins in a page. Scrolling is now at parity with single process browsers in my testing.
This works by making painting of child windows asynchronous. The key is to send, not post, the custom message. This allows the redraw message to jump the queue and get dispatched before other queued [Windows, IPC] messages, which eliminates painting artifacts.
Now that painting is asynchronous, I had to take out the SWP_NOREDRAW flag to DeferWindowPos. While it's slightly faster without it, the visual aritfacts are noticable.