Add character properties checking and a comment about problematic characters

normal/charset.c

@@ -16,6 +16,25 @@
  *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+/*
+  Current problems with Unicode rendering: 
+  - B and BN bidi type characters (ignored)
+  - Mc type characters with combining class 0 (treated as non-combining)
+  - Mn type characters with combining class 0 (treated as non-combining)
+  - Me type characters with combining class 0 (treated as non-combining)
+  - Cf type characters (ignored)
+  - Cc type characters (ignored)
+  - Line-breaking rules (e.g. Zs type characters)
+  - Indic languages
+  - Arabic shaping
+  - Zl and Zp characters
+  - Combining characters of types 7, 8, 9, 21, 27, 28, 29, 30, 31,
+  32, 33, 34, 35, 36, 84, 91, 103, 107, 118, 122, 129, 130, 132,
+  218, 224, 226, 233, 234
+  - Private use surrogates
+  - Private use characters (not really a problem)
+ */
+
 /* Convert a (possibly null-terminated) UTF-8 string of at most SRCSIZE
    bytes (if SRCSIZE is -1, it is ignored) in length to a UTF-16 string.
    Return the number of characters converted. DEST must be able to hold