Win32: Fix invalid hat bit mask being accepted

It is reportedly possible to get opposing directions of an XInput DPad
bit mask set simultaneously with some controllers.

This commit ensures that those values are not passed on to other parts
of GLFW.

This commit is based on the PR #2291 by @ PeterJohnson but with the
following changes:
 - moved XInput-specific special case to XInput implementation
 - attempt to preserve data by only masking out the invalid axis
 - admin (credit, changelog, commit message)

Closes #2291

CONTRIBUTORS.md

@@ -102,6 +102,7 @@ video tutorials.
  - JannikGM
  - Erik S. V. Jansson
  - jjYBdx4IL
+ - Peter Johnson
  - Toni Jovanoski
  - Arseny Kapoulkine
  - Cem Karan