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.

(cherry picked from commit 9a3664b4a9)

README.md

@@ -132,6 +132,8 @@ information on what to include when reporting a bug.
    non-printable keys (#1598)
  - [X11] Bugfix: Function keys were mapped to `GLFW_KEY_UNKNOWN` for some layout
    combinaitons (#1598)
+ - [X11] Bugfix: Keys pressed simultaneously with others were not always
+   reported (#1112,#1415,#1472,#1616)
  - [Wayland] Bugfix: Repeated keys could be reported with `NULL` window (#1704)
 
 
@@ -295,8 +297,10 @@ skills.
  - Eddie Ringle
  - Max Risuhin
  - Jorge Rodriguez
+ - Luca Rood
  - Ed Ropple
  - Aleksey Rybalkin
+ - Mikko Rytkönen
  - Riku Salminen
  - Brandon Schaefer
  - Sebastian Schuberth