XInput2: Accept input from keyboards other than the first master

XInput2 was created to support multiple pointer/keyboard pairs (often
called MPX for multi-pointer X).  Dolphin's XInput2 implementation has
always supported MPX by creating a KeyboardMouse object per master
pointer.  Since commit bbb12a7, Dolphin's keyboard state is filtered by
the output of XQueryKeymap.  As a core X function, XQueryKeymap queries
"the" keyboard, which by default is the first master keyboard.  As a
result, Dolphin will ignore keys pressed on other master keyboards
unless the first master is simultaneously pressing the same keys.

XInput2 doesn't provide a function to query the keyboard state.  There
is no XIQueryKeymap and the current state is not a member of the
XIKeyClassInfo returned by XIQueryDevice.  Instead, XInput2 allows a
master pointer to be nominated as "the" pointer on a per-client basis,
with "the" keyboard automatically becoming the associated master
keyboard.  The "documentation" [1] says passing None for the window is
only for debugging purposes, but it is documented in the
XISetClientPointer man page and seems to be the only way to query
keyboards beyond the first.

With this commit, Dolphin correctly reads keys from keyboards other than
the first master keyboard.  To test, use the xinput command-line utility
to create a master pointer and reattach a keyboard to the associated
master keyboard.

[1]: https://who-t.blogspot.com/2009/07/xi2-recipes-part-6.html
     (the XInput2 developer's blog)

Source/Core/InputCommon/ControllerInterface/Xlib/XInput2.cpp

@@ -166,6 +166,10 @@ KeyboardMouse::KeyboardMouse(Window window, int opcode, int pointer, int keyboar
   name = std::string(pointer_device->name);
   XIFreeDeviceInfo(pointer_device);
 
+  // Tell core X functions which keyboard is "the" keyboard for this
+  // X connection.
+  XISetClientPointer(m_display, None, pointer_deviceid);
+
   {
     unsigned char mask_buf[(XI_LASTEVENT + 7) / 8] = {};
     XISetMask(mask_buf, XI_ButtonPress);
@@ -387,6 +391,10 @@ void KeyboardMouse::UpdateInput()
   // KeyRelease and FocusOut events are sometimes not received.
   // Cycling Alt-Tab and landing on the same window results in a stuck "Alt" key.
   // Unpressed keys are released here.
+  // Because we called XISetClientPointer in the constructor, XQueryKeymap
+  // will return the state of the associated keyboard, even if it isn't the
+  // first master keyboard.  (XInput2 doesn't provide a function to query
+  // keyboard state.)
   std::array<char, 32> keyboard;
   XQueryKeymap(m_display, keyboard.data());
   for (size_t i = 0; i != keyboard.size(); ++i)