x11/wayland: Fix signal handling while blocking in WaitEventTimeout()

Add a new flag to avoid suppressing EINTR in SDL_IOReady(). Pass the
flag in WaitEventTimeout() to ensure that a SIGINT will wake up
SDL_WaitEvent() without another event coming in.

src/core/unix/SDL_poll.h

@@ -28,6 +28,7 @@
 
 #define SDL_IOR_READ           0x1
 #define SDL_IOR_WRITE          0x2
+#define SDL_IOR_NO_RETRY       0x4
 
 extern int SDL_IOReady(int fd, int flags, int timeoutMS);