Win32: Fix scancode and key for Alt+PrtSc events

Alt+PrtSc emits a different scancode than just PrtSc.  Since the GLFW
API assumes each key corresponds to only one scancode, this cannot be
added to the keycodes array.

Instead we replace the scancode at the point of entry.

Fixes #1993

(cherry picked from commit 03cfe957e7)

CONTRIBUTORS.md

@@ -69,6 +69,7 @@ video tutorials.
  - Ryan C. Gordon
  - Stephen Gowen
  - Kovid Goyal
+ - Kevin Grandemange
  - Eloi Marín Gratacós
  - Stefan Gustavson
  - Andrew Gutekanst

README.md

@@ -123,7 +123,8 @@ information on what to include when reporting a bug.
 
 ## Changelog
 
-There is nothing here yet.
+ - [Win32] Bugfix: `Alt+PrtSc` would emit `GLFW_KEY_UNKNOWN` and a different
+   scancode than `PrtSc` (#1993)
 
 
 ## Contact

src/win32_window.c

@@ -748,6 +748,10 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
                 scancode = MapVirtualKeyW((UINT) wParam, MAPVK_VK_TO_VSC);
             }
 
+            // HACK: Alt+PrtSc has a different scancode than just PrtSc
+            if (scancode == 0x54)
+                scancode = 0x137;
+
             key = _glfw.win32.keycodes[scancode];
 
             // The Ctrl keys require special handling