X11 scancode mapping cleanup

* Consolidated scancode mapping tables into a single location for all backends
* Verified that the xfree86_scancode_table2 is largely identical to the Linux scancode table
* Updated the Linux scancode table with the latest kernel keycodes (still unmapped)
* Route X11 keysym -> scancode mapping through the linux scancode table (which a few hand-written exceptions), which will allow mappings to automatically get picked up as they are added in the Linux scancode table
* Disabled verbose reporting of missing keysym mappings, we have enough data for now

include/SDL_scancode.h

@@ -345,6 +345,11 @@ typedef enum
      *  \name Usage page 0x0C
      *
      *  These values are mapped from usage page 0x0C (USB consumer page).
+     *  See https://usb.org/sites/default/files/hut1_2.pdf
+     *
+     *  There are way more keys in the spec than we can represent in the
+     *  current scancode range, so pick the ones that commonly come up in
+     *  real world usage.
      */
     /* @{ */