Fix ImTextCountUtf8BytesFromChar and ImTextCharToUtf8, these APIs ass…

…ume the input is an unicode code point, not UTF-16

imgui.cpp

@@ -1746,11 +1746,15 @@ static inline int ImTextCharToUtf8(char* buf, int buf_size, unsigned int c)
         buf[1] = (char)(0x80 + (c & 0x3f));
         return 2;
     }
-    if (c >= 0xdc00 && c < 0xe000)
+    if (c < 0x10000)
     {
-        return 0;
+        if (buf_size < 3) return 0;
+        buf[0] = (char)(0xe0 + (c >> 12));
+        buf[1] = (char)(0x80 + ((c>> 6) & 0x3f));
+        buf[2] = (char)(0x80 + ((c ) & 0x3f));
+        return 3;
     }
-    if (c >= 0xd800 && c < 0xdc00)
+    if (c <= 0x10FFFF)
     {
         if (buf_size < 4) return 0;
         buf[0] = (char)(0xf0 + (c >> 18));
@@ -1759,14 +1763,8 @@ static inline int ImTextCharToUtf8(char* buf, int buf_size, unsigned int c)
         buf[3] = (char)(0x80 + ((c ) & 0x3f));
         return 4;
     }
-    //else if (c < 0x10000)
-    {
-        if (buf_size < 3) return 0;
-        buf[0] = (char)(0xe0 + (c >> 12));
-        buf[1] = (char)(0x80 + ((c>> 6) & 0x3f));
-        buf[2] = (char)(0x80 + ((c ) & 0x3f));
-        return 3;
-    }
+    // Invalid code point, the max unicode is 0x10FFFF
+    return 0;
 }
 
 // Not optimal but we very rarely use this function.
@@ -1780,8 +1778,8 @@ static inline int ImTextCountUtf8BytesFromChar(unsigned int c)
 {
     if (c < 0x80) return 1;
     if (c < 0x800) return 2;
-    if (c >= 0xdc00 && c < 0xe000) return 0;
-    if (c >= 0xd800 && c < 0xdc00) return 4;
+    if (c < 0x10000) return 3;
+    if (c <= 0x10FFFF) return 4;
     return 3;
 }