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

tools/convttf.c

@@ -91,7 +91,7 @@ static int eid = DEFAULT_ENCODING_ID;
 static FT_UShort nocmap;
 
 int pct                     = 0; /* display ttc table if it is not zero. */
-FT_Long         max_char    = 65535;
+FT_Long         max_char    = 1024*1024;  /* Unicode maxes out at 2**20 */
 int             pixel_size  = 15;
 FT_Long         start_char  = 0;
 FT_Long         limit_char;