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

CONTRIBUTORS.md

@@ -70,6 +70,7 @@ video tutorials.
  - Ryan C. Gordon
  - Stephen Gowen
  - Kovid Goyal
+ - Kevin Grandemange
  - Eloi Marín Gratacós
  - Stefan Gustavson
  - Andrew Gutekanst

README.md

@@ -210,6 +210,8 @@ information on what to include when reporting a bug.
  - [Win32] Bugfix: The default restored window position was lost when creating a maximized
    window
  - [Win32] Bugfix: `glfwMaximizeWindow` would make a hidden window visible
+ - [Win32] Bugfix: `Alt+PrtSc` would emit `GLFW_KEY_UNKNOWN` and a different
+   scancode than `PrtSc` (#1993)
  - [Cocoa] Added support for `VK_EXT_metal_surface` (#1619)
  - [Cocoa] Added locating the Vulkan loader at runtime in an application bundle
  - [Cocoa] Moved main menu creation to GLFW initialization time (#1649)

src/win32_window.c

@@ -752,6 +752,10 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
                 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