Wayland provides the prepare_read()/read_events() family of APIs for
reading from the display fd in a deadlock-free manner across multiple
threads in a multi-threaded application. Let's use those instead of
trying to roll our own solution using a mutex.
This fixes an issue where a call to SDL_GL_SwapWindow() doesn't swap
buffers if it happens to collide with SDL_PumpEvents() in the main
thread. It also allows coexistence with other code or toolkits in
our process that may want read and dispatch events themselves.
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