Limit length of read operation in ImageFont._load_pilfont_data()

[radarhere]

A few ImageFont._load_pilfont_data() suggestions.

1. Remove an unnecessary split()
   

   Pillow/src/PIL/ImageFont.py

   Line 132 in 31eee6e

   file.readline().split(b";")

2. Check image mode before reading from file, to return the error faster.
3. Rather than potentially reading an entire line,
   

   Pillow/src/PIL/ImageFont.py

   Line 129 in 31eee6e

   if file.readline() != b"PILfont\n":

   
   we can limit the length of the read operation by just using read(8).