Win32: Fix non-BMP Unicode codepoint input

Supplementary Plane codepoints from WM_CHAR and WM_SYSCHAR messages were
reported as UTF-16 surrogate pairs.

Related to #1635.

(cherry picked from commit 6ce2070392)

src/win32_platform.h

@@ -318,6 +318,8 @@ typedef struct _GLFWwindowWin32
 
     // The last received cursor position, regardless of source
     int                 lastCursorPosX, lastCursorPosY;
+    // The last recevied high surrogate when decoding pairs of UTF-16 messages
+    WCHAR               highSurrogate;
 
 } _GLFWwindowWin32;