improve centering on highlighted slides

example/1-hi.txt

···
 HI THERE
 
-Welcome to ${GRN}shlide${RST}. Here are a few bullet points:
+Welcome to ${GRN}shlide${RST}. ${YLW}Here${RST} are a few bullet points:
 
 - first point
 - second point
     * sub point
-    * another sub point
+    * ${RED}another${RST} sub point

shlide

···
 }
 
 longest_line() {
-    max=0 IFS=
+    max=0 
+    local IFS=
     while read -r line; do
-        if [ "${#line}" -gt "$max" ]; then max="${#line}"; fi
+        l=$(ansi_filter $(colorify "$line"))
+        echo "$l" >> log.txt
+        if [ "${#l}" -gt "$max" ]; then max="${#l}"; fi
     done < "$1"
     printf '%s\n' "$max"
 }