InputText: amends: fixes overwrite mode handling. (7925)

Note that there is a bug (not new) when undoing restores the cursor at a wrong position.

imstb_textedit.h

@@ -747,8 +747,11 @@ static void stb_textedit_text(IMSTB_TEXTEDIT_STRING* str, STB_TexteditState* sta
       return;
 
    if (state->insert_mode && !STB_TEXT_HAS_SELECTION(state) && state->cursor < STB_TEXTEDIT_STRINGLEN(str)) {
-      stb_text_makeundo_replace(str, state, state->cursor, 1, 1);
-      STB_TEXTEDIT_DELETECHARS(str, state->cursor, 1);
+      // [DEAR IMGUI] This assume that text contains a single character.
+      // [DEAR IMGUI] FIXME: When undoing the restored cursor position is wrong, but it's not a new bug.
+      int prev_char_len = IMSTB_TEXTEDIT_GETNEXTCHARINDEX(str, state->cursor) - state->cursor;
+      stb_text_makeundo_replace(str, state, state->cursor, prev_char_len, text_len);
+      STB_TEXTEDIT_DELETECHARS(str, state->cursor, prev_char_len);
       if (STB_TEXTEDIT_INSERTCHARS(str, state->cursor, text, text_len)) {
          state->cursor += text_len;
          state->has_preferred_x = 0;