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

lib/rbcodec/metadata/id3tags.c

@@ -1092,9 +1092,9 @@ retry_with_limit:
 
                     if (!parse_as_utf8(tag, &bytesread))
                     {
-                        /* UTF-8 could potentially be 3 times larger */
+                        /* UTF-8 could potentially be 4 times larger */
                         /* so we need to create a new buffer         */
-                        int utf8_size = (3 * bytesread);
+                        int utf8_size = (4 * bytesread);
                         if (utf8_size > ID3V2_BUF_SIZE)
                         {
                             //limit stack allocation to avoid stack overflow