Add glfwGetKeyScancode

Allows retrieval of platform scancode from GLFW_KEY_*.  Implemented for
Win32, Cocoa and X11.  Stubs for Mir and Wayland.

Closes #830.

docs/input.dox

@@ -223,6 +223,20 @@ ignored.  This matches the behavior of the key callback, meaning the callback
 arguments can always be passed unmodified to this function.
 
 
+@subsection input_key_scancode Key scancodes
+
+If you need the platform dependent scancode for any given key, you can query
+it with @ref glfwGetKeyScancode.
+
+@code
+const short int scancode = glfwGetKeyScancode(GLFW_KEY_X);
+set_key_mapping(scancode, swap_weapons);
+@encode
+
+If the key is `GLFW_KEY_UNKNOWN` or does not exist on the keyboard this
+method will return `-1`.
+
+
 @section input_mouse Mouse input
 
 Mouse input comes in many forms, including cursor motion, button presses and

docs/news.dox

@@ -5,6 +5,12 @@
 @section news_33 New features in 3.3
 
 
+@subsection new_33_keyscancode Platform dependent scancodes
+
+GLFW now supports querying the platform dependent scancode of any key with
+@ref glfwGetKeyScancode.
+
+
 @section news_32 New features in 3.2