wgl: Fix running headless with Waffle on Windows

The message handling is broken when running headless. Normally, we get a WM_PAINT right when the window is shown which causes the test to display and then exit (for -auto usage, which is the only way they should be run when headless). But when running in a disconnected session like in a CI environment, you don't get a WM_PAINT - and in fact you don't get any messages, meaning that the test hangs in GetMessage. Switching that to PeekMessage prevents the hang, but the test still doesn't actually display, unless we set need_redisplay to true upon creation without relying on a WM_PAINT message or a post_redisplay from the test.

With these two changes, we can run Waffle in headless environments.