Support Unicode Full Repertoire cmap tables.

For example, "Apple Color Emoji.ttc" has such a cmap. After this commit,
we can render its glyph for "U+0001F1E6 REGIONAL INDICATOR SYMBOL LETTER
A", which is clearly outside the Unicode BMP.

Note that this only lets us render black-and-white glyphs for out-of-BMP
runes. Rendering color glyphs (i.e. emoji) remains TODO.

Updates #45

truetype/truetype.go

@@ -57,7 +57,8 @@ const (
 	// A 32-bit encoding consists of a most-significant 16-bit Platform ID and a
 	// least-significant 16-bit Platform Specific ID. The magic numbers are
 	// specified at https://www.microsoft.com/typography/otspec/name.htm
-	unicodeEncoding         = 0x00000003 // PID = 0 (Unicode), PSID = 3 (Unicode 2.0)
+	unicodeEncodingBMPOnly  = 0x00000003 // PID = 0 (Unicode), PSID = 3 (Unicode 2.0 BMP Only)
+	unicodeEncodingFull     = 0x00000004 // PID = 0 (Unicode), PSID = 4 (Unicode 2.0 Full Repertoire)
 	microsoftSymbolEncoding = 0x00030000 // PID = 3 (Microsoft), PSID = 0 (Symbol)
 	microsoftUCS2Encoding   = 0x00030001 // PID = 3 (Microsoft), PSID = 1 (UCS-2)
 	microsoftUCS4Encoding   = 0x0003000a // PID = 3 (Microsoft), PSID = 10 (UCS-4)
@@ -142,7 +143,7 @@ func parseSubtables(table []byte, name string, offset, size int, pred func([]byt
 		pidPsid := u32(table, offset)
 		// We prefer the Unicode cmap encoding. Failing to find that, we fall
 		// back onto the Microsoft cmap encoding.
-		if pidPsid == unicodeEncoding {
+		if pidPsid == unicodeEncodingBMPOnly || pidPsid == unicodeEncodingFull {
 			bestOffset, bestPID, ok = offset, pidPsid>>16, true
 			break