internals: Support characters beyond the first unicode plane (WIP)

We used 16-bit variables to store the 'character code' everywhere but
this won't let us represent anything beyond U+FFFF.

This patch changes those variables to a custom type that can be 32 or 16
bits depending on the build, and adjusts numerous internal APIs and
datastructures to match.  This includes:

 * utf8decode() and friends
 * on-screen keyboard
 * font manipulation, caching, rendering, and generation
 * VFAT code parses and generates utf16 dirents
 * WIN32 simulator reads and writes utf16 filenames

Note that this patch doesn't _enable_ >16bit unicode support; a followup
patch will turn that on for appropriate targets.

Known bugs:

  * Native players in 32-bit unicode mode generate mangled filename
    entries if they include UTF16 surrogate codepoints.  Root cause
    is unclear, and may reside in core dircache code.

Needs testing on:

 * windows simulator (16bit+32bit)

Change-Id: I193a00fe2a11a4181ddc82df2d71be52bf00b6e6

apps/plugins/lua/rocklib.c

@@ -141,6 +141,7 @@ RB_WRAP(touchscreen_mode)
 
 #endif
 
+// XXX this may be broken with 32-bit ucschar_t
 RB_WRAP(kbd_input)
 {
     /*kbd_input(text, layout)*
@@ -168,7 +169,7 @@ RB_WRAP(kbd_input)
         layout = NULL;
     }
 
-    if(!rb->kbd_input(buffer, LUAL_BUFFERSIZE, (unsigned short *)layout))
+    if(!rb->kbd_input(buffer, LUAL_BUFFERSIZE, (ucschar_t *)layout))
     {
         luaL_addstring(&b, buffer);
         luaL_pushresult(&b);