This MR introduces the driver mechanisms to handle dynamic events from the Wayland compositor, using wl_output events as the guiding use case (i.e., we want to update the win32u display settings when the host settings change).
In this design we create a dedicated thread to read and dispatch Wayland events received from the compositor. If a Wayland event handler wants some code to be run in the context of a particular HWND's thread, it can add an internal event to a custom queue we have for each (GUI enabled) thread. The ProcessEvents driver callback processes internal events from that queue. In order to wake up waiting threads we use a pipe to notify about new internal events, with the read end acting as the thread's host queue fd. This is similar to how winemac.drv works.
We use the aforementioned mechanisms to queue win32u display device updates to the desktop window thread. Since there are many pieces that need to fall into place, this MR gradually reaches the final design:
1. We first introduce the dedicated read/dispatch thread and handle events (and also display device updates if in the desktop process) in that thread. 2. We ensure access to Wayland output information is thread-safe and consistent (since in step 3 we will need to access it from a different thread). 3. We finally introduce per-thread internal event queues and, if we are in the desktop process, queue the display device update to the desktop window thread internal event queue. Note that the main portion of the wl_output event code is still handled in the dedicated read/dispatch thread.