X11: Query and keep track of Xkb group index

For users with multiple keyboard layouts configured, glfwGetKeyName
works fine only with the primary layout.  Switching layouts results in
changing the group index.  This commit querries the current group index
when initializing keyboard input and keeps track of any change to it.

As a result the scancode -> keyname mapping may change while the program
is running (needs to be documented).

Fixes #1462.
Closes #1528.

(cherry picked from commit 36f9080)

src/x11_init.c

@@ -54,7 +54,7 @@ static int translateKeyCode(int scancode)
         // Note: This way we always force "NumLock = ON", which is intentional
         // since the returned key code should correspond to a physical
         // location.
-        keySym = XkbKeycodeToKeysym(_glfw.x11.display, scancode, 0, 1);
+        keySym = XkbKeycodeToKeysym(_glfw.x11.display, scancode, _glfw.x11.xkb.group, 1);
         switch (keySym)
         {
             case XK_KP_0:           return GLFW_KEY_KP_0;
@@ -76,7 +76,7 @@ static int translateKeyCode(int scancode)
 
         // Now try primary keysym for function keys (non-printable keys)
         // These should not depend on the current keyboard layout
-        keySym = XkbKeycodeToKeysym(_glfw.x11.display, scancode, 0, 0);
+        keySym = XkbKeycodeToKeysym(_glfw.x11.display, scancode, _glfw.x11.xkb.group, 0);
     }
     else
     {
@@ -659,6 +659,14 @@ static GLFWbool initExtensions(void)
             if (supported)
                 _glfw.x11.xkb.detectable = GLFW_TRUE;
         }
+
+        _glfw.x11.xkb.group = 0;
+        XkbStateRec state;
+        if (XkbGetState(_glfw.x11.display, XkbUseCoreKbd, &state) == Success)
+        {
+            XkbSelectEventDetails(_glfw.x11.display, XkbUseCoreKbd, XkbStateNotify, XkbAllStateComponentsMask, XkbGroupStateMask);
+            _glfw.x11.xkb.group = (unsigned int)state.group;
+        }
     }
 
 #if defined(__CYGWIN__)

src/x11_platform.h

@@ -323,13 +323,14 @@ typedef struct _GLFWlibraryX11
     } randr;
 
     struct {
-        GLFWbool    available;
-        GLFWbool    detectable;
-        int         majorOpcode;
-        int         eventBase;
-        int         errorBase;
-        int         major;
-        int         minor;
+        GLFWbool     available;
+        GLFWbool     detectable;
+        int          majorOpcode;
+        int          eventBase;
+        int          errorBase;
+        int          major;
+        int          minor;
+        unsigned int group;
     } xkb;
 
     struct {

src/x11_window.c

@@ -1180,6 +1180,15 @@ static void processEvent(XEvent *event)
         }
     }
 
+    if (_glfw.x11.xkb.available && event->type == _glfw.x11.xkb.eventBase)
+    {
+        if (((XkbEvent *)event)->any.xkb_type == XkbStateNotify &&
+            ((XkbEvent *)event)->state.changed & XkbGroupStateMask)
+        {
+            _glfw.x11.xkb.group = ((XkbEvent *)event)->state.group;
+        }
+    }
+
     if (event->type == GenericEvent)
     {
         if (_glfw.x11.xi.available)
@@ -2780,7 +2789,7 @@ const char* _glfwPlatformGetScancodeName(int scancode)
     if (!_glfw.x11.xkb.available)
         return NULL;
 
-    const KeySym keysym = XkbKeycodeToKeysym(_glfw.x11.display, scancode, 0, 0);
+    const KeySym keysym = XkbKeycodeToKeysym(_glfw.x11.display, scancode, _glfw.x11.xkb.group, 0);
     if (keysym == NoSymbol)
         return NULL;