pcsx2: call XInitThreads at init

X11 isn't thread safe by default. It make sense in 1990 but it is ugly nowadays.

The trick is that is must called before any X11 function. So the only
safe place is at the start of the main.  Pcsx2App::OnInit() is the
sooner that I've found.

pcsx2/CMakeLists.txt

@@ -662,6 +662,7 @@ set(pcsx2FinalLibs
     ${ZLIB_LIBRARIES}
     ${AIO_LIBRARIES}
     ${GCOV_LIBRARIES}
+    ${X11_LIBRARIES}
 )
 
 if(BUILTIN_GS)

pcsx2/gui/AppInit.cpp

@@ -424,8 +424,20 @@ protected:
 	}
 };
 
+#ifdef __unix__
+#include <X11/Xlib.h>
+#endif
+
 bool Pcsx2App::OnInit()
 {
+#ifdef __unix__
+	// By default X11 isn't thread safe
+	// Typically it avoid a crash on Mesa when glthread is enabled on DRI2
+	//
+	// I guess it could be removed once we migrate to Wayland (post 2020)
+	XInitThreads();
+#endif
+
 	EnableAllLogging();
 	Console.WriteLn("Interface is initializing.  Entering Pcsx2App::OnInit!");