Some synthetic key messages come with a scancode of zero, causing them
to be translate to GLFW_KEY_UNKNOWN.  This fills in the missing scancode
by translating the provided virtual key.

Rather than further complicate a single-use function, its body is merged
into the key message handler.

Fixes #1623.

(cherry picked from commit 789a2bcb3985a882944f3ae8dc6aa8aa22a23494)