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