Reduce number of bytes read for PCX header

[radarhere]

1. PcxImagePlugin initially reads 128 bytes
   

   Pillow/src/PIL/PcxImagePlugin.py

   Line 57 in 8e5a15b

   s = self.fp.read(128)

   
   But the header only uses up to 68 bytes.
   

   Pillow/src/PIL/PcxImagePlugin.py

   Line 73 in 8e5a15b

   provided_stride = i16(s, 66)

   
   So the number of bytes can be reduced.

2. After seeking to the end of the file for one type of header, PcxImagePlugin seeks back to the start.
   

   Pillow/src/PIL/PcxImagePlugin.py

   Lines 96 to 105 in 8e5a15b

   	s = self.fp.read(769)
   	if len(s) == 769 and s[0] == 12:
   	# check if the palette is linear grayscale
   	for i in range(256):
   	if s[i * 3 + 1 : i * 3 + 4] != o8(i) * 3:
   	mode = rawmode = "P"
   	break
   	if mode == "P":
   	self.palette = ImagePalette.raw("RGB", s[1:])
   	self.fp.seek(128)

This is unnecessary, since load() will seek to the tile offset.