Fixed filtering of events duplicated by XIM.

Duplicate events cannot be filtered by the return value of XFilterEvent,
as that discards dead key events on some IMs (ibus), nor by its inverse,
as that discards all key events on other IMs (?).  This solution is
based on the workaround in SDL2 and takes advantage of the identical
time of the duplicate events.

Fixes #548.
Closes #554.
Closes #571.

src/x11_platform.h

@@ -96,6 +96,12 @@ typedef struct _GLFWwindowX11
     // The last position the cursor was warped to by GLFW
     int             warpPosX, warpPosY;
 
+    // The information from the last KeyPress event
+    struct {
+        unsigned int keycode;
+        Time         time;
+    } last;
+
 } _GLFWwindowX11;