X11: Fix detection of key events duplicated by XIM

Background: The IM will filter out key events, instead sending exact
duplicate events that are not filtered.  It does not send these for
every event, however, so the duplicate events cannot be relied on for
key input.  Instead we need to identify and discard them.  Since they
are identical, they have the same timestamp as the originals.

The previous duplicate event detection would consume unrelated key
events if the keys were pressed simultaneously, as it only tracked
a single timestamp.

This fixes that issue for any combination of keys, at the expense of
a 1 KB array per GLFW window.

This fix is a stopgap until explicit IME support is done.

Based on #1472 by @LucaRood.

Fixes #1112.
Fixes #1415.
Fixes #1616.
Fixes #1663.
Closes #1472.

src/x11_platform.h

@@ -402,8 +402,9 @@ typedef struct _GLFWwindowX11
     // The last position the cursor was warped to by GLFW
     int             warpCursorPosX, warpCursorPosY;
 
-    // The time of the last KeyPress event
-    Time            lastKeyTime;
+    // The time of the last KeyPress event per keycode, for discarding
+    // duplicate key events generated for some keys by ibus
+    Time            keyPressTimes[256];
 
 } _GLFWwindowX11;